From: Paul Hilfinger <Hilfinger@cs.berkeley.edu>
Date: Fri, 23 Jul 2010 02:08:10 +0000 (-0700)
Subject: Allow specification of semantic predicates.
X-Git-Tag: v2.7.90~673
X-Git-Url: https://git.saurik.com/bison.git/commitdiff_plain/ca2a6d1587f421a6c58b9f80cc8d6b983c9a62dc

Allow specification of semantic predicates.

These changes allow users to prefix an action with %? to indicate that it
is a semantic predicate---an expression that is evaluated immediately (not
deferred, even in GLR nondeterministic mode) and causes a syntax error if
false.  In GLR parsers, this has the effect of killing one of a set of
split-off parses, just as would an ordinary syntax error.

Changelog:

    * NEWS: Describe new semantic-predicate feature.
    * data/c.m4 (b4_predicate_case): New definition.
    * data/java.m4 (b4_predicate_case): New definition.
    * data/glr.c (yyimmediate): Add definition.
    (yydoAction): Remove comment, now obsolete.
    Do YY_REDUCE_PRINT here.
    (yyglrReduce): Alter comment to indicate that semantic values
    need not be deferred.
    Remove YY_REDUCE_PRINT from here; done in yydoAction.
    (yyprocessOneStack): Pass immediate flag.
    Delete stacks rejected by predicates in newly split-off parsers.
    Change handling of yyerr so that only current stack gets deleted
    when semantic predicate fails.
    (yyfillin): Don't crash if a semantic value is unresolved (as may
    happen in predicate rules).
    Copy lr state as well in debugging mode.
    Update comment on setting of yysval to include yyloc as well.
    (yy_reduce_print): Add yynormal argument.  Perform fillin properly.
    Report unresolved RHS values.
    (yyimmediate): New table.
    * src/gram.h (struct rule): Add is_predicate field.
    * src/output.c (user_actions_output): Use b4_predicate_case for
    predicates.
    (prepare_symbols): Output yyimmediate.
    * src/scan-gram.l: Add %? token, SC_PREDICATE state.
    * src/scan-code.l (code_props_rule_action_init): Add is_predicate
    argument.
    * src/scan-code.h (struct code_props): Add is_predicate field.
    (code_props_rule_action_init): New interface.
    * src/parse-gram.y (%?{...}): New token.
    (rhs): Add %?{...} rule.
    * src/parse-gram.c: Regenerate.
    * src/parse-gram.h: Regenerate.
    * src/reader.c (grammar_current_rule_action_append): Add
    immediate argument.
    (grammar_midrule_action): Use new interface for
    code_props_rule_action_init.
    (grammar_current_rule_action_append): Ditto.
    (packgram): Transfer is_predicate value.
    * src/reader.h (grammar_current_rule_action_append): New interface.
    * doc/bison.texinfo: Document semantic predicates (%?).

    * data/glr.c (yylhsNonterm, yyisDefaultedState,yyDefaultAction)
    (yygetLRActions,yynewGLRStackItem,yyaddDeferredAction,yyinitStateSet)
    (yyinitGLRStack,yyexpandGLRStack,yyupdateSplit,yymarkStackDeleted)
    (yyundeleteLastStack,yyglrShift,yyglrShiftDefer,yydoAction,yyglrReduce)
    (yyidenticalOptions,yymergeOptionSets,yyresolveStates,yyresolveAction)
    (yyresolveLocations,yyresolveValue,yyreducePrint): Update parameter
    names in comments and mention all parameters.
    (struct yyGLRState): Fix description of yyposn field.
    (yyresolveLocations): Correct comment so as not to imply action when
    yyn1==0.
---

diff --git a/ChangeLog b/ChangeLog
index 29a119c7..9a35e816 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,58 @@
+2010-07-23  Paul Hilfinger  <hilfingr@EECS.Berkeley.EDU>
+
+	* NEWS: Describe new semantic-predicate feature.
+        * data/c.m4 (b4_predicate_case): New definition.
+        * data/java.m4 (b4_predicate_case): New definition.
+        * data/glr.c (yyimmediate): Add definition.
+        (yydoAction): Remove comment, now obsolete.
+        Do YY_REDUCE_PRINT here.
+        (yyglrReduce): Alter comment to indicate that semantic values
+        need not be deferred.
+        Remove YY_REDUCE_PRINT from here; done in yydoAction.
+        (yyprocessOneStack): Pass immediate flag.
+        Delete stacks rejected by predicates in newly split-off parsers.
+        Change handling of yyerr so that only current stack gets deleted
+        when semantic predicate fails.
+        (yyfillin): Don't crash if a semantic value is unresolved (as may
+        happen in predicate rules).
+        Copy lr state as well in debugging mode.
+        Update comment on setting of yysval to include yyloc as well.
+        (yy_reduce_print): Add yynormal argument.  Perform fillin properly.
+        Report unresolved RHS values.
+        (yyimmediate): New table.
+        * src/gram.h (struct rule): Add is_predicate field.
+        * src/output.c (user_actions_output): Use b4_predicate_case for
+        predicates.
+        (prepare_symbols): Output yyimmediate.
+        * src/scan-gram.l: Add %? token, SC_PREDICATE state.
+        * src/scan-code.l (code_props_rule_action_init): Add is_predicate
+        argument.
+        * src/scan-code.h (struct code_props): Add is_predicate field.
+        (code_props_rule_action_init): New interface.
+        * src/parse-gram.y (%?{...}): New token.
+        (rhs): Add %?{...} rule.
+        * src/parse-gram.c: Regenerate.
+        * src/parse-gram.h: Regenerate.
+        * src/reader.c (grammar_current_rule_action_append): Add
+        immediate argument.
+        (grammar_midrule_action): Use new interface for
+        code_props_rule_action_init.
+        (grammar_current_rule_action_append): Ditto.
+        (packgram): Transfer is_predicate value.
+        * src/reader.h (grammar_current_rule_action_append): New interface.
+        * doc/bison.texinfo: Document semantic predicates (%?).
+    
+        * data/glr.c (yylhsNonterm, yyisDefaultedState,yyDefaultAction)
+        (yygetLRActions,yynewGLRStackItem,yyaddDeferredAction,yyinitStateSet)
+        (yyinitGLRStack,yyexpandGLRStack,yyupdateSplit,yymarkStackDeleted)
+        (yyundeleteLastStack,yyglrShift,yyglrShiftDefer,yydoAction,yyglrReduce)
+        (yyidenticalOptions,yymergeOptionSets,yyresolveStates,yyresolveAction)
+        (yyresolveLocations,yyresolveValue,yyreducePrint): Update parameter
+        names in comments and mention all parameters.
+        (struct yyGLRState): Fix description of yyposn field.
+        (yyresolveLocations): Correct comment so as not to imply action when
+        yyn1==0.
+	
 2010-06-17  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Update from GFDL GFDL 1.2 to 1.3.
diff --git a/NEWS b/NEWS
index e5ebfe4b..878756bd 100644
--- a/NEWS
+++ b/NEWS
@@ -51,6 +51,15 @@ Bison News
   use of the %error-verbose directive is deprecated in favor of
   %define parse.error "verbose".
 
+** Semantic predicates
+
+  The new, experimental, semantic-predicate feature allows actions of
+  the form %?{ BOOLEAN-EXPRESSION }, which cause syntax errors (as for
+  YYERROR) if the expression evaluates to 0, and are evaluated immediately
+  in GLR parsers, rather than being deferred.  The result is that they
+  allow the programmer to prune possible parses based on the values of
+  runtime expressions.
+
 * Changes in version 2.5 (????-??-??):
 
 ** Named References Support
diff --git a/data/c.m4 b/data/c.m4
index 9e7b3193..db685c9a 100644
--- a/data/c.m4
+++ b/data/c.m4
@@ -417,6 +417,15 @@ b4_syncline([@oline@], [@ofile@])
     break;])
 
 
+# b4_predicate_case(LABEL, CONDITIONS)
+# ------------------------------------
+m4_define([b4_predicate_case],
+[  case $1:
+    if (! ($2)) YYERROR;
+b4_syncline([@oline@], [@ofile@])
+    break;])
+
+
 # b4_yydestruct_generate(FUNCTION-DECLARATOR)
 # -------------------------------------------
 # Generate the "yydestruct" function, which declaration is issued using
diff --git a/data/glr.c b/data/glr.c
index 3c6ea8d7..b72aef75 100644
--- a/data/glr.c
+++ b/data/glr.c
@@ -402,6 +402,13 @@ static const ]b4_int_type_for([b4_merger])[ yymerger[] =
   ]b4_merger[
 };
 
+/* YYIMMEDIATE[RULE-NUM] -- True iff rule #RULE-NUM is not to be deferred, as
+   in the case of predicates.  */
+static const yybool yyimmediate[] =
+{
+  ]b4_immediate[
+};
+
 /* YYCONFLP[YYPACT[STATE-NUM]] -- Pointer into YYCONFL of start of
    list of conflicting reductions corresponding to action entry for
    state STATE-NUM in yytable.  0 means no conflicts.  The list in
@@ -693,7 +700,7 @@ struct yyGLRState {
   yyStateNum yylrState;
   /** Preceding state in this stack */
   yyGLRState* yypred;
-  /** Source position of the first token produced by my symbol */
+  /** Source position of the last token produced by my symbol */
   size_t yyposn;
   union {
     /** First in a chain of alternative reductions producing the
@@ -806,9 +813,16 @@ yyfillin (yyGLRStackItem *yyvsp, int yylow0, int yylow1)
   s = yyvsp[yylow0].yystate.yypred;
   for (i = yylow0-1; i >= yylow1; i -= 1)
     {
-      YYASSERT (s->yyresolved);
-      yyvsp[i].yystate.yyresolved = yytrue;
-      yyvsp[i].yystate.yysemantics.yysval = s->yysemantics.yysval;
+#if YYDEBUG
+      yyvsp[i].yystate.yylrState = s->yylrState;
+#endif
+      yyvsp[i].yystate.yyresolved = s->yyresolved;
+      if (s->yyresolved)
+	yyvsp[i].yystate.yysemantics.yysval = s->yysemantics.yysval;
+      else
+	/* The effect of using yysval or yyloc (in an immediate rule) is
+         * undefined.  */
+	yyvsp[i].yystate.yysemantics.yyfirstVal = NULL;
       yyvsp[i].yystate.yyloc = s->yyloc;
       s = yyvsp[i].yystate.yypred = s->yypred;
     }
@@ -948,7 +962,7 @@ yydestroyGLRState (char const *yymsg, yyGLRState *yys]b4_user_formals[)
     }
 }
 
-/** Left-hand-side symbol for rule #RULE.  */
+/** Left-hand-side symbol for rule #YYRULE.  */
 static inline yySymbol
 yylhsNonterm (yyRuleNum yyrule)
 {
@@ -958,7 +972,7 @@ yylhsNonterm (yyRuleNum yyrule)
 #define yypact_value_is_default(yystate) \
   ]b4_table_value_equals([[pact]], [[yystate]], [b4_pact_ninf])[
 
-/** True iff LR state STATE has only a default reduction (regardless
+/** True iff LR state YYSTATE has only a default reduction (regardless
  *  of token).  */
 static inline yybool
 yyisDefaultedState (yyStateNum yystate)
@@ -966,7 +980,7 @@ yyisDefaultedState (yyStateNum yystate)
   return yypact_value_is_default (yypact[yystate]);
 }
 
-/** The default reduction for STATE, assuming it has one.  */
+/** The default reduction for YYSTATE, assuming it has one.  */
 static inline yyRuleNum
 yydefaultAction (yyStateNum yystate)
 {
@@ -981,8 +995,8 @@ yydefaultAction (yyStateNum yystate)
  *    R < 0:  Reduce on rule -R.
  *    R = 0:  Error.
  *    R > 0:  Shift to state R.
- *  Set *CONFLICTS to a pointer into yyconfl to 0-terminated list of
- *  conflicting reductions.
+ *  Set *YYCONFLICTS to a pointer into yyconfl to a 0-terminated list
+ *  of conflicting reductions.
  */
 static inline void
 yygetLRActions (yyStateNum yystate, int yytoken,
@@ -1031,9 +1045,10 @@ yyisErrorAction (int yyaction)
 
 				/* GLRStates */
 
-/** Return a fresh GLRStackItem.  Callers should call
- * YY_RESERVE_GLRSTACK afterwards to make sure there is sufficient
- * headroom.  */
+/** Return a fresh GLRStackItem in YYSTACKP.  The item is an LR state
+ *  if YYISSTATE, and otherwise a semantic option.  Callers should call
+ *  YY_RESERVE_GLRSTACK afterwards to make sure there is sufficient
+ *  headroom.  */
 
 static inline yyGLRStackItem*
 yynewGLRStackItem (yyGLRStack* yystackp, yybool yyisState)
@@ -1046,9 +1061,9 @@ yynewGLRStackItem (yyGLRStack* yystackp, yybool yyisState)
 }
 
 /** Add a new semantic action that will execute the action for rule
- *  RULENUM on the semantic values in RHS to the list of
- *  alternative actions for STATE.  Assumes that RHS comes from
- *  stack #K of *STACKP. */
+ *  YYRULE on the semantic values in YYRHS to the list of
+ *  alternative actions for YYSTATE.  Assumes that YYRHS comes from
+ *  stack #YYK of *YYSTACKP. */
 static void
 yyaddDeferredAction (yyGLRStack* yystackp, size_t yyk, yyGLRState* yystate,
 		     yyGLRState* yyrhs, yyRuleNum yyrule)
@@ -1073,7 +1088,7 @@ yyaddDeferredAction (yyGLRStack* yystackp, size_t yyk, yyGLRState* yystate,
 
 				/* GLRStacks */
 
-/** Initialize SET to a singleton set containing an empty stack.  */
+/** Initialize YYSET to a singleton set containing an empty stack.  */
 static yybool
 yyinitStateSet (yyGLRStateSet* yyset)
 {
@@ -1099,8 +1114,8 @@ static void yyfreeStateSet (yyGLRStateSet* yyset)
   YYFREE (yyset->yylookaheadNeeds);
 }
 
-/** Initialize STACK to a single empty stack, with total maximum
- *  capacity for all stacks of SIZE.  */
+/** Initialize *YYSTACKP to a single empty stack, with total maximum
+ *  capacity for all stacks of YYSIZE.  */
 static yybool
 yyinitGLRStack (yyGLRStack* yystackp, size_t yysize)
 {
@@ -1122,7 +1137,7 @@ yyinitGLRStack (yyGLRStack* yystackp, size_t yysize)
 # define YYRELOC(YYFROMITEMS,YYTOITEMS,YYX,YYTYPE) \
   &((YYTOITEMS) - ((YYFROMITEMS) - (yyGLRStackItem*) (YYX)))->YYTYPE
 
-/** If STACK is expandable, extend it.  WARNING: Pointers into the
+/** If *YYSTACKP is expandable, extend it.  WARNING: Pointers into the
     stack from outside should be considered invalid after this call.
     We always expand when there are 1 or fewer items left AFTER an
     allocation, so that we can avoid having external pointers exist
@@ -1192,9 +1207,9 @@ yyfreeGLRStack (yyGLRStack* yystackp)
   yyfreeStateSet (&yystackp->yytops);
 }
 
-/** Assuming that S is a GLRState somewhere on STACK, update the
- *  splitpoint of STACK, if needed, so that it is at least as deep as
- *  S.  */
+/** Assuming that YYS is a GLRState somewhere on *YYSTACKP, update the
+ *  splitpoint of *YYSTACKP, if needed, so that it is at least as deep as
+ *  YYS.  */
 static inline void
 yyupdateSplit (yyGLRStack* yystackp, yyGLRState* yys)
 {
@@ -1202,7 +1217,7 @@ yyupdateSplit (yyGLRStack* yystackp, yyGLRState* yys)
     yystackp->yysplitPoint = yys;
 }
 
-/** Invalidate stack #K in STACK.  */
+/** Invalidate stack #YYK in *YYSTACKP.  */
 static inline void
 yymarkStackDeleted (yyGLRStack* yystackp, size_t yyk)
 {
@@ -1211,8 +1226,8 @@ yymarkStackDeleted (yyGLRStack* yystackp, size_t yyk)
   yystackp->yytops.yystates[yyk] = NULL;
 }
 
-/** Undelete the last stack that was marked as deleted.  Can only be
-    done once after a deletion, and only when all other stacks have
+/** Undelete the last stack in *YYSTACKP that was marked as deleted.  Can
+    only be done once after a deletion, and only when all other stacks have
     been deleted.  */
 static void
 yyundeleteLastStack (yyGLRStack* yystackp)
@@ -1261,8 +1276,9 @@ yyremoveDeletes (yyGLRStack* yystackp)
     }
 }
 
-/** Shift to a new state on stack #K of STACK, corresponding to LR state
- * LRSTATE, at input position POSN, with (resolved) semantic value SVAL.  */
+/** Shift to a new state on stack #YYK of *YYSTACKP, corresponding to LR
+ * state YYLRSTATE, at input position YYPOSN, with (resolved) semantic
+ * value *YYVALP and source location *YYLOCP.  */
 static inline void
 yyglrShift (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
 	    size_t yyposn,
@@ -1281,7 +1297,7 @@ yyglrShift (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
   YY_RESERVE_GLRSTACK (yystackp);
 }
 
-/** Shift stack #K of YYSTACK, to a new state corresponding to LR
+/** Shift stack #YYK of *YYSTACKP, to a new state corresponding to LR
  *  state YYLRSTATE, at input position YYPOSN, with the (unresolved)
  *  semantic value of YYRHS under the action for YYRULE.  */
 static inline void
@@ -1301,11 +1317,51 @@ yyglrShiftDefer (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
   yyaddDeferredAction (yystackp, yyk, yynewState, yyrhs, yyrule);
 }
 
-/** Pop the symbols consumed by reduction #RULE from the top of stack
- *  #K of STACK, and perform the appropriate semantic action on their
+#if !YYDEBUG
+# define YY_REDUCE_PRINT(Args)
+#else
+# define YY_REDUCE_PRINT(Args)		\
+do {					\
+  if (yydebug)				\
+    yy_reduce_print Args;		\
+} while (YYID (0))
+
+/*----------------------------------------------------------------------.
+| Report that stack #YYK of *YYSTACKP is going to be reduced by YYRULE. |
+`----------------------------------------------------------------------*/
+
+/*ARGSUSED*/ static inline void
+yy_reduce_print (int yynormal, yyGLRStackItem* yyrhsVals, size_t yyk, yyRuleNum yyrule]b4_user_formals[)
+{
+  int yynrhs = yyrhsLength (yyrule);
+  int yyi;
+]b4_parse_param_use[]dnl
+[  YYFPRINTF (stderr, "Reducing stack %lu by rule %d (line %lu):\n",
+	     (unsigned long int) yyk, yyrule - 1,
+	     (unsigned long int) yyrline[yyrule]);
+  if (! yynormal)
+    yyfillin (yyrhsVals, 1, -yynrhs);
+  /* The symbols being reduced.  */
+  for (yyi = 0; yyi < yynrhs; yyi++)
+    {
+      YYFPRINTF (stderr, "   $%d = ", yyi + 1);
+      yy_symbol_print (stderr,
+		       yystos[yyrhsVals[yyi - yynrhs + 1].yystate.yylrState],
+		       &yyrhsVals[yyi - yynrhs + 1].yystate.yysemantics.yysval
+		       ]b4_locations_if([, &]b4_rhs_location(yynrhs, yyi + 1))[]dnl
+		       b4_user_args[);
+      if (!yyrhsVals[yyi - yynrhs + 1].yystate.yyresolved)
+        YYFPRINTF (stderr, " (unresolved)");
+      YYFPRINTF (stderr, "\n");
+    }
+}
+#endif
+
+/** Pop the symbols consumed by reduction #YYRULE from the top of stack
+ *  #YYK of *YYSTACKP, and perform the appropriate semantic action on their
  *  semantic values.  Assumes that all ambiguities in semantic values
- *  have been previously resolved.  Set *VALP to the resulting value,
- *  and *LOCP to the computed location (if any).  Return value is as
+ *  have been previously resolved.  Set *YYVALP to the resulting value,
+ *  and *YYLOCP to the computed location (if any).  Return value is as
  *  for userAction.  */
 static inline YYRESULTTAG
 yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
@@ -1321,15 +1377,12 @@ yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
       yystackp->yynextFree -= yynrhs;
       yystackp->yyspaceLeft += yynrhs;
       yystackp->yytops.yystates[0] = & yystackp->yynextFree[-1].yystate;
+      YY_REDUCE_PRINT ((1, yyrhs, yyk, yyrule]b4_user_args[));
       return yyuserAction (yyrule, yynrhs, yyrhs,
 			   yyvalp, yylocp, yystackp]b4_user_args[);
     }
   else
     {
-      /* At present, doAction is never called in nondeterministic
-       * mode, so this branch is never taken.  It is here in
-       * anticipation of a future feature that will allow immediate
-       * evaluation of selected actions in nondeterministic mode.  */
       int yyi;
       yyGLRState* yys;
       yyGLRStackItem yyrhsVals[YYMAXRHS + YYMAXLEFT + 1];
@@ -1345,60 +1398,21 @@ yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
 	}
       yyupdateSplit (yystackp, yys);
       yystackp->yytops.yystates[yyk] = yys;
+      YY_REDUCE_PRINT ((0, yyrhsVals + YYMAXRHS + YYMAXLEFT - 1, yyk, yyrule]b4_user_args[));
       return yyuserAction (yyrule, yynrhs, yyrhsVals + YYMAXRHS + YYMAXLEFT - 1,
 			   yyvalp, yylocp, yystackp]b4_user_args[);
     }
 }
 
-#if !YYDEBUG
-# define YY_REDUCE_PRINT(Args)
-#else
-# define YY_REDUCE_PRINT(Args)		\
-do {					\
-  if (yydebug)				\
-    yy_reduce_print Args;		\
-} while (YYID (0))
-
-/*----------------------------------------------------------.
-| Report that the RULE is going to be reduced on stack #K.  |
-`----------------------------------------------------------*/
-
-/*ARGSUSED*/ static inline void
-yy_reduce_print (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule]b4_user_formals[)
-{
-  int yynrhs = yyrhsLength (yyrule);
-  yybool yynormal __attribute__ ((__unused__)) =
-    (yystackp->yysplitPoint == NULL);
-  yyGLRStackItem* yyvsp = (yyGLRStackItem*) yystackp->yytops.yystates[yyk];
-  int yylow = 1;
-  int yyi;
-]b4_parse_param_use[]dnl
-[  YYFPRINTF (stderr, "Reducing stack %lu by rule %d (line %lu):\n",
-	     (unsigned long int) yyk, yyrule - 1,
-	     (unsigned long int) yyrline[yyrule]);
-  /* The symbols being reduced.  */
-  for (yyi = 0; yyi < yynrhs; yyi++)
-    {
-      YYFPRINTF (stderr, "   $%d = ", yyi + 1);
-      yy_symbol_print (stderr,
-                       ]yystos@{b4_rhs_data(yynrhs, yyi + 1).yylrState@}[,
-		       &]b4_rhs_value(yynrhs, yyi + 1)[
-		       ]b4_locations_if([, &]b4_rhs_location(yynrhs, yyi + 1))[]dnl
-		       b4_user_args[);
-      YYFPRINTF (stderr, "\n");
-    }
-}
-#endif
-
-/** Pop items off stack #K of STACK according to grammar rule RULE,
+/** Pop items off stack #YYK of *YYSTACKP according to grammar rule YYRULE,
  *  and push back on the resulting nonterminal symbol.  Perform the
- *  semantic action associated with RULE and store its value with the
- *  newly pushed state, if FORCEEVAL or if STACK is currently
+ *  semantic action associated with YYRULE and store its value with the
+ *  newly pushed state, if YYFORCEEVAL or if *YYSTACKP is currently
  *  unambiguous.  Otherwise, store the deferred semantic action with
  *  the new state.  If the new state would have an identical input
  *  position, LR state, and predecessor to an existing state on the stack,
- *  it is identified with that existing state, eliminating stack #K from
- *  the STACK.  In this case, the (necessarily deferred) semantic value is
+ *  it is identified with that existing state, eliminating stack #YYK from
+ *  *YYSTACKP.  In this case, the semantic value is
  *  added to the options for the existing state's semantic value.
  */
 static inline YYRESULTTAG
@@ -1409,12 +1423,19 @@ yyglrReduce (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
 
   if (yyforceEval || yystackp->yysplitPoint == NULL)
     {
+      YYRESULTTAG yyflag;
       YYSTYPE yysval;
       YYLTYPE yyloc;
 
-      YY_REDUCE_PRINT ((yystackp, yyk, yyrule]b4_user_args[));
-      YYCHK (yydoAction (yystackp, yyk, yyrule, &yysval,
-			 &yyloc]b4_user_args[));
+      yyflag = yydoAction (yystackp, yyk, yyrule, &yysval,
+			   &yyloc]b4_user_args[);
+      if (yyflag == yyerr)
+        {
+	  YYDPRINTF ((stderr, "Parse on stack %lu rejected by rule #%d.\n",
+		     (unsigned long int) yyk, yyrule - 1));
+        }
+      if (yyflag != yyok)
+        return yyflag;      
       YY_SYMBOL_PRINT ("-> $$ =", yyr1[yyrule], &yysval, &yyloc);
       yyglrShift (yystackp, yyk,
 		  yyLRgotoState (yystackp->yytops.yystates[yyk]->yylrState,
@@ -1508,7 +1529,7 @@ yysplitStack (yyGLRStack* yystackp, size_t yyk)
   return yystackp->yytops.yysize-1;
 }
 
-/** True iff Y0 and Y1 represent identical options at the top level.
+/** True iff YYY0 and YYY1 represent identical options at the top level.
  *  That is, they represent the same rule applied to RHS symbols
  *  that produce the same terminal symbols.  */
 static yybool
@@ -1530,8 +1551,8 @@ yyidenticalOptions (yySemanticOption* yyy0, yySemanticOption* yyy1)
     return yyfalse;
 }
 
-/** Assuming identicalOptions (Y0,Y1), destructively merge the
- *  alternative semantic values for the RHS-symbols of Y1 and Y0.  */
+/** Assuming identicalOptions (YYY0,YYY1), destructively merge the
+ *  alternative semantic values for the RHS-symbols of YYY1 and YYY0.  */
 static void
 yymergeOptionSets (yySemanticOption* yyy0, yySemanticOption* yyy1)
 {
@@ -1612,11 +1633,11 @@ static YYRESULTTAG yyresolveValue (yyGLRState* yys,
 				   yyGLRStack* yystackp]b4_user_formals[);
 
 
-/** Resolve the previous N states starting at and including state S.  If result
- *  != yyok, some states may have been left unresolved possibly with empty
- *  semantic option chains.  Regardless of whether result = yyok, each state
- *  has been left with consistent data so that yydestroyGLRState can be invoked
- *  if necessary.  */
+/** Resolve the previous YYN states starting at and including state YYS
+ *  on *YYSTACKP. If result != yyok, some states may have been left
+ *  unresolved possibly with empty semantic option chains.  Regardless
+ *  of whether result = yyok, each state has been left with consistent
+ *  data so that yydestroyGLRState can be invoked if necessary.  */
 static YYRESULTTAG
 yyresolveStates (yyGLRState* yys, int yyn,
 		 yyGLRStack* yystackp]b4_user_formals[)
@@ -1631,9 +1652,10 @@ yyresolveStates (yyGLRState* yys, int yyn,
   return yyok;
 }
 
-/** Resolve the states for the RHS of OPT, perform its user action, and return
- *  the semantic value and location.  Regardless of whether result = yyok, all
- *  RHS states have been destroyed (assuming the user action destroys all RHS
+/** Resolve the states for the RHS of YYOPT on *YYSTACKP, perform its
+ *  user action, and return the semantic value and location in *YYVALP
+ *  and *YYLOCP.  Regardless of whether result = yyok, all RHS states
+ *  have been destroyed (assuming the user action destroys all RHS
  *  semantic values if invoked).  */
 static YYRESULTTAG
 yyresolveAction (yySemanticOption* yyopt, yyGLRStack* yystackp,
@@ -1743,9 +1765,9 @@ yyreportAmbiguity (yySemanticOption* yyx0,
   return yyabort;
 }
 
-/** Starting at and including state S1, resolve the location for each of the
- *  previous N1 states that is unresolved.  The first semantic option of a state
- *  is always chosen.  */
+/** Resolve the locations for each of the YYN1 states in *YYSTACKP,
+ *  ending at YYS1.  Has no effect on previously resolved states.
+ *  The first semantic option of a state is always chosen.  */
 static void
 yyresolveLocations (yyGLRState* yys1, int yyn1,
 		    yyGLRStack *yystackp]b4_user_formals[)
@@ -1801,11 +1823,12 @@ yyresolveLocations (yyGLRState* yys1, int yyn1,
     }
 }
 
-/** Resolve the ambiguity represented in state S, perform the indicated
- *  actions, and set the semantic value of S.  If result != yyok, the chain of
- *  semantic options in S has been cleared instead or it has been left
- *  unmodified except that redundant options may have been removed.  Regardless
- *  of whether result = yyok, S has been left with consistent data so that
+/** Resolve the ambiguity represented in state YYS in *YYSTACKP,
+ *  perform the indicated actions, and set the semantic value of YYS.
+ *  If result != yyok, the chain of semantic options in YYS has been
+ *  cleared instead or it has been left unmodified except that
+ *  redundant options may have been removed.  Regardless of whether
+ *  result = yyok, YYS has been left with consistent data so that
  *  yydestroyGLRState can be invoked if necessary.  */
 static YYRESULTTAG
 yyresolveValue (yyGLRState* yys, yyGLRStack* yystackp]b4_user_formals[)
@@ -1950,6 +1973,7 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
   int yyaction;
   const short int* yyconflicts;
   yyRuleNum yyrule;
+  YYRESULTTAG yyflag;
 
   while (yystackp->yytops.yystates[yyk] != NULL)
     {
@@ -1969,11 +1993,21 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 	      yymarkStackDeleted (yystackp, yyk);
 	      return yyok;
 	    }
-	  YYCHK (yyglrReduce (yystackp, yyk, yyrule, yyfalse]b4_user_args[));
+	  yyflag = yyglrReduce (yystackp, yyk, yyrule, yyimmediate[yyrule]]b4_user_args[);
+          if (yyflag == yyerr)
+            {
+	      YYDPRINTF ((stderr, "Stack %lu dies (predicate failure or explicit user error).\n",
+			  (unsigned long int) yyk));
+	      yymarkStackDeleted (yystackp, yyk);
+	      return yyok;
+	    }
+	  if (yyflag != yyok)
+            return yyflag;
 	}
       else
 	{
 	  yySymbol yytoken;
+
 	  yystackp->yytops.yylookaheadNeeds[yyk] = yytrue;
 	  if (yychar == YYEMPTY)
 	    {
@@ -1996,14 +2030,25 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 
 	  while (*yyconflicts != 0)
 	    {
+	      YYRESULTTAG yyflag;
 	      size_t yynewStack = yysplitStack (yystackp, yyk);
 	      YYDPRINTF ((stderr, "Splitting off stack %lu from %lu.\n",
 			  (unsigned long int) yynewStack,
 			  (unsigned long int) yyk));
-	      YYCHK (yyglrReduce (yystackp, yynewStack,
-				  *yyconflicts, yyfalse]b4_user_args[));
-	      YYCHK (yyprocessOneStack (yystackp, yynewStack,
-					yyposn]b4_pure_args[));
+	      yyflag = yyglrReduce (yystackp, yynewStack,
+				    *yyconflicts,
+				    yyimmediate[*yyconflicts]]b4_user_args[);
+              if (yyflag == yyok)
+	        YYCHK (yyprocessOneStack (yystackp, yynewStack,
+					  yyposn]b4_pure_args[));
+	      else if (yyflag == yyerr)
+		{
+		  YYDPRINTF ((stderr, "Stack %lu dies.\n", 
+			      (unsigned long int) yynewStack));
+		  yymarkStackDeleted (yystackp, yynewStack);
+		}
+	      else
+		return yyflag;
 	      yyconflicts += 1;
 	    }
 
@@ -2017,8 +2062,19 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 	      break;
 	    }
 	  else
-	    YYCHK (yyglrReduce (yystackp, yyk, -yyaction,
-				yyfalse]b4_user_args[));
+            {
+	      yyflag = yyglrReduce (yystackp, yyk, -yyaction,
+		  		    yyimmediate[-yyaction]]b4_user_args[);
+	      if (yyflag == yyerr)
+                {
+	          YYDPRINTF ((stderr, "Stack %lu dies (predicate failure or explicit user error).\n",
+			      (unsigned long int) yyk));
+	          yymarkStackDeleted (yystackp, yyk);
+                  break;
+		}
+	      else if (yyflag != yyok)
+                return yyflag;
+            }
 	}
     }
   return yyok;
@@ -2249,7 +2305,6 @@ yyrecoverSyntaxError (yyGLRStack* yystackp]b4_user_formals[)
     }									     \
   } while (YYID (0))
 
-
 /*----------.
 | yyparse.  |
 `----------*/
diff --git a/data/java.m4 b/data/java.m4
index 781a7df2..32ff2156 100644
--- a/data/java.m4
+++ b/data/java.m4
@@ -169,6 +169,13 @@ m4_define([b4_case], [  case $1:
   break;
     ])
 
+# b4_predicate_case(LABEL, CONDITIONS)
+# ------------------------------------
+m4_define([b4_predicate_case], [  case $1:
+     if (! ($2)) YYERROR;
+    break;
+    ])
+
 
 ## ---------------- ##
 ## Default values.  ##
diff --git a/doc/bison.texinfo b/doc/bison.texinfo
index 3a4ee845..6d9d1be2 100644
--- a/doc/bison.texinfo
+++ b/doc/bison.texinfo
@@ -136,6 +136,7 @@ Writing @acronym{GLR} Parsers
 * Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
 * Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
 * GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Semantic Predicates::    Controlling a parse with arbitrary computations.
 * Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
 
 Examples
@@ -758,6 +759,7 @@ merged result.
 * Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
 * Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
 * GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Semantic Predicates::    Controlling a parse with arbitrary computations.
 * Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
 @end menu
 
@@ -1163,6 +1165,65 @@ In a deferred semantic action, its effect is undefined.
 Also, see @ref{Location Default Action, ,Default Action for Locations}, which
 describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers.
 
+@node Semantic Predicates
+@subsection Controlling a Parse with Arbitrary Predicates
+@findex %?
+@cindex Semantic predicates in @acronym{GLR} parsers
+
+In addition to the @code{%dprec} and @code{%merge} directives,
+@acronym{GLR} parsers
+allow you to reject parses on the basis of arbitrary computations executed
+in user code, without having Bison treat this rejection as an error
+if there are alternative parses. (This feature is experimental and may
+evolve.  We welcome user feedback.)  For example,
+
+@smallexample
+widget :
+          %?@{ new_syntax @} "widget" id new_args   @{ $$ = f($3, $4); @}
+       |  %?@{ !new_syntax @} "widget" id old_args  @{ $$ = f($3, $4); @}
+       ;
+@end smallexample
+
+@noindent
+is one way to allow the same parser to handle two different syntaxes for 
+widgets.  The clause preceded by @code{%?} is treated like an ordinary
+action, except that its text is treated as an expression and is always
+evaluated immediately (even when in nondeterministic mode).  If the 
+expression yields 0 (false), the clause is treated as a syntax error,
+which, in a nondeterministic parser, causes the stack in which it is reduced 
+to die.  In a deterministic parser, it acts like YYERROR.
+
+As the example shows, predicates otherwise look like semantic actions, and
+therefore you must be take them into account when determining the numbers
+to use for denoting the semantic values of right-hand side symbols.
+Predicate actions, however, have no defined value, and may not be given
+labels.
+
+There is a subtle difference between semantic predicates and ordinary
+actions in nondeterministic mode, since the latter are deferred.
+For example, we could try to rewrite the previous example as 
+
+@smallexample
+widget :
+          @{ if (!new_syntax) YYERROR; @} "widget" id new_args  @{ $$ = f($3, $4); @}
+       |  @{ if (new_syntax) YYERROR; @} "widget" id old_args   @{ $$ = f($3, $4); @}
+       ;
+@end smallexample
+
+@noindent
+(reversing the sense of the predicate tests to cause an error when they are
+false).  However, this
+does @emph{not} have the same effect if @code{new_args} and @code{old_args}
+have overlapping syntax.
+Since the mid-rule actions testing @code{new_syntax} are deferred, 
+a @acronym{GLR} parser first encounters the unresolved ambiguous reduction 
+for cases where @code{new_args} and @code{old_args} recognize the same string
+@emph{before} performing the tests of @code{new_syntax}.  It therefore
+reports an error.
+
+Finally, be careful in writing predicates: deferred actions have not been
+evaluated, so that using them in a predicate will have undefined effects.
+
 @node Compiler Requirements
 @subsection Considerations when Compiling @acronym{GLR} Parsers
 @cindex @code{inline}
@@ -10610,6 +10671,19 @@ file.  @xref{Grammar Outline, ,Outline of a Bison
 Grammar}.
 @end deffn
 
+@deffn {Directive} %?@{@var{expression}@}
+Predicate actions.  This is a type of action clause that may appear in
+rules. The expression is evaluated, and if false, causes a syntax error.  In
+@acronym{GLR} parsers during nondeterministic operation,  
+this silently causes an alternative parse to die.  During deterministic
+operation, it is the same as the effect of YYERROR.
+@xref{Semantic Predicates}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+@end deffn
+
 @deffn {Construct} /*@dots{}*/
 Comment delimiters, as in C.
 @end deffn
diff --git a/src/gram.h b/src/gram.h
index 1b750fd1..85792d6f 100644
--- a/src/gram.h
+++ b/src/gram.h
@@ -194,6 +194,7 @@ typedef struct
 
   location location;
   bool useful;
+  bool is_predicate;
 
   const char *action;
   location action_location;
diff --git a/src/output.c b/src/output.c
index fa448a6f..6b29e5c0 100644
--- a/src/output.c
+++ b/src/output.c
@@ -204,7 +204,7 @@ prepare_symbols (void)
 
 /*----------------------------------------------------------------.
 | Prepare the muscles related to the rules: r1, r2, rline, dprec, |
-| merger.                                                         |
+| merger, immediate.                                              |
 `----------------------------------------------------------------*/
 
 static void
@@ -215,6 +215,7 @@ prepare_rules (void)
   unsigned int *r2 = xnmalloc (nrules, sizeof *r2);
   int *dprec = xnmalloc (nrules, sizeof *dprec);
   int *merger = xnmalloc (nrules, sizeof *merger);
+  int *immediate = xnmalloc (nrules, sizeof *immediate);
 
   rule_number r;
   for (r = 0; r < nrules; ++r)
@@ -229,6 +230,8 @@ prepare_rules (void)
       dprec[r] = rules[r].dprec;
       /* Merger-function index (GLR).  */
       merger[r] = rules[r].merger;
+      /* Immediate reduction flags (GLR).  */
+      immediate[r] = rules[r].is_predicate;
     }
 
   muscle_insert_unsigned_int_table ("rline", rline, 0, 0, nrules);
@@ -236,6 +239,7 @@ prepare_rules (void)
   muscle_insert_unsigned_int_table ("r2", r2, 0, 0, nrules);
   muscle_insert_int_table ("dprec", dprec, 0, 0, nrules);
   muscle_insert_int_table ("merger", merger, 0, 0, nrules);
+  muscle_insert_int_table ("immediate", immediate, 0, 0, nrules);
 
   MUSCLE_INSERT_INT ("rules_number", nrules);
   MUSCLE_INSERT_INT ("max_left_semantic_context", max_left_semantic_context);
@@ -349,8 +353,9 @@ user_actions_output (FILE *out)
   for (r = 0; r < nrules; ++r)
     if (rules[r].action)
       {
-	fprintf (out, "b4_case(%d, [b4_syncline(%d, ", r + 1,
-		 rules[r].action_location.start.line);
+	fprintf (out, "b4_%scase(%d, [b4_syncline(%d, ",
+		 rules[r].is_predicate ? "predicate_" : "",
+		 r + 1, rules[r].action_location.start.line);
 	escaped_output (out, rules[r].action_location.start.file);
 	fprintf (out, ")\n[    %s]])\n\n", rules[r].action);
       }
diff --git a/src/parse-gram.c b/src/parse-gram.c
index 4fef5a2f..e5713416 100644
--- a/src/parse-gram.c
+++ b/src/parse-gram.c
@@ -1,8 +1,9 @@
-/* A Bison parser, made by GNU Bison 2.4.483-4ad39-dirty.  */
+/* A Bison parser, made by GNU Bison 2.4.516-804e.  */
 
 /* Implementation for Bison's Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2010 Free Software Foundation, Inc.
+   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
+   2007, 2008, 2009, 2010 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -44,7 +45,7 @@
 #define YYBISON 1
 
 /* Bison version.  */
-#define YYBISON_VERSION "2.4.483-4ad39-dirty"
+#define YYBISON_VERSION "2.4.516-804e"
 
 /* Skeleton name.  */
 #define YYSKELETON_NAME "yacc.c"
@@ -76,7 +77,8 @@
 #line 1 "src/parse-gram.y"
 /* Bison Grammar Parser                             -*- C -*-
 
-   Copyright (C) 2002-2010 Free Software Foundation, Inc.
+   Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
+   Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -127,7 +129,7 @@ static void gram_error (location const *, char const *);
 static char const *char_name (char);
 
 /* Line 252 of yacc.c  */
-#line 131 "src/parse-gram.c"
+#line 133 "src/parse-gram.c"
 
 /* Enabling traces.  */
 #ifndef YYDEBUG
@@ -149,7 +151,7 @@ static char const *char_name (char);
 
 /* "%code requires" blocks.  */
 /* Line 272 of yacc.c  */
-#line 201 "src/parse-gram.y"
+#line 202 "src/parse-gram.y"
 
 # ifndef PARAM_TYPE
 #  define PARAM_TYPE
@@ -164,7 +166,7 @@ static char const *char_name (char);
 
 
 /* Line 272 of yacc.c  */
-#line 168 "src/parse-gram.c"
+#line 170 "src/parse-gram.c"
 
 /* Tokens.  */
 #ifndef YYTOKENTYPE
@@ -211,21 +213,22 @@ static char const *char_name (char);
      PERCENT_VERBOSE = 293,
      PERCENT_YACC = 294,
      BRACED_CODE = 295,
-     BRACKETED_ID = 296,
-     CHAR = 297,
-     EPILOGUE = 298,
-     EQUAL = 299,
-     ID = 300,
-     ID_COLON = 301,
-     PERCENT_PERCENT = 302,
-     PIPE = 303,
-     PROLOGUE = 304,
-     SEMICOLON = 305,
-     TAG = 306,
-     TAG_ANY = 307,
-     TAG_NONE = 308,
-     PERCENT_PARAM = 309,
-     PERCENT_UNION = 310
+     BRACED_PREDICATE = 296,
+     BRACKETED_ID = 297,
+     CHAR = 298,
+     EPILOGUE = 299,
+     EQUAL = 300,
+     ID = 301,
+     ID_COLON = 302,
+     PERCENT_PERCENT = 303,
+     PIPE = 304,
+     PROLOGUE = 305,
+     SEMICOLON = 306,
+     TAG = 307,
+     TAG_ANY = 308,
+     TAG_NONE = 309,
+     PERCENT_PARAM = 310,
+     PERCENT_UNION = 311
    };
 #endif
 /* Tokens.  */
@@ -268,21 +271,22 @@ static char const *char_name (char);
 #define PERCENT_VERBOSE 293
 #define PERCENT_YACC 294
 #define BRACED_CODE 295
-#define BRACKETED_ID 296
-#define CHAR 297
-#define EPILOGUE 298
-#define EQUAL 299
-#define ID 300
-#define ID_COLON 301
-#define PERCENT_PERCENT 302
-#define PIPE 303
-#define PROLOGUE 304
-#define SEMICOLON 305
-#define TAG 306
-#define TAG_ANY 307
-#define TAG_NONE 308
-#define PERCENT_PARAM 309
-#define PERCENT_UNION 310
+#define BRACED_PREDICATE 296
+#define BRACKETED_ID 297
+#define CHAR 298
+#define EPILOGUE 299
+#define EQUAL 300
+#define ID 301
+#define ID_COLON 302
+#define PERCENT_PERCENT 303
+#define PIPE 304
+#define PROLOGUE 305
+#define SEMICOLON 306
+#define TAG 307
+#define TAG_ANY 308
+#define TAG_NONE 309
+#define PERCENT_PARAM 310
+#define PERCENT_UNION 311
 
 
 
@@ -291,7 +295,7 @@ static char const *char_name (char);
 typedef union YYSTYPE
 {
 /* Line 277 of yacc.c  */
-#line 87 "src/parse-gram.y"
+#line 88 "src/parse-gram.y"
 
   assoc assoc;
   char *code;
@@ -304,13 +308,13 @@ typedef union YYSTYPE
   unsigned char character;
 
 /* Line 277 of yacc.c  */
-#line 225 "src/parse-gram.y"
+#line 226 "src/parse-gram.y"
 
   param_type param;
 
 
 /* Line 277 of yacc.c  */
-#line 314 "src/parse-gram.c"
+#line 318 "src/parse-gram.c"
 } YYSTYPE;
 # define YYSTYPE_IS_TRIVIAL 1
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
@@ -334,10 +338,10 @@ typedef struct YYLTYPE
 /* Copy the second part of user declarations.  */
 
 /* Line 327 of yacc.c  */
-#line 338 "src/parse-gram.c"
+#line 342 "src/parse-gram.c"
 /* Unqualified %code blocks.  */
 /* Line 328 of yacc.c  */
-#line 55 "src/parse-gram.y"
+#line 56 "src/parse-gram.y"
 
   static int current_prec = 0;
   static location current_lhs_location;
@@ -352,7 +356,7 @@ typedef struct YYLTYPE
   #define YYTYPE_UINT8 uint_fast8_t
 
 /* Line 328 of yacc.c  */
-#line 214 "src/parse-gram.y"
+#line 215 "src/parse-gram.y"
 
   /** Add a lex-param and/or a parse-param.
    *
@@ -365,7 +369,7 @@ typedef struct YYLTYPE
 
 
 /* Line 328 of yacc.c  */
-#line 369 "src/parse-gram.c"
+#line 373 "src/parse-gram.c"
 
 #ifdef short
 # undef short
@@ -582,21 +586,21 @@ union yyalloc
 /* YYFINAL -- State number of the termination state.  */
 #define YYFINAL  3
 /* YYLAST -- Last index in YYTABLE.  */
-#define YYLAST   162
+#define YYLAST   163
 
 /* YYNTOKENS -- Number of terminals.  */
-#define YYNTOKENS  56
+#define YYNTOKENS  57
 /* YYNNTS -- Number of nonterminals.  */
 #define YYNNTS  36
 /* YYNRULES -- Number of rules.  */
-#define YYNRULES  109
+#define YYNRULES  110
 /* YYNSTATES -- Number of states.  */
-#define YYNSTATES  148
+#define YYNSTATES  149
 
 /* YYTRANSLATE[YYX] -- Symbol number corresponding to YYX as returned
    by yylex, with out-of-bounds checking.  */
 #define YYUNDEFTOK  2
-#define YYMAXUTOK   310
+#define YYMAXUTOK   311
 
 #define YYTRANSLATE(YYX)						\
   ((unsigned int) (YYX) <= YYMAXUTOK ? yytranslate[YYX] : YYUNDEFTOK)
@@ -636,24 +640,25 @@ static const yytype_uint8 yytranslate[] =
       25,    26,    27,    28,    29,    30,    31,    32,    33,    34,
       35,    36,    37,    38,    39,    40,    41,    42,    43,    44,
       45,    46,    47,    48,    49,    50,    51,    52,    53,    54,
-      55
+      55,    56
 };
 
 #if YYDEBUG
   /* YYRLINEYYN -- Source line where rule number YYN was defined.    */
 static const yytype_uint16 yyrline[] =
 {
-       0,   250,   250,   258,   260,   264,   265,   275,   279,   284,
-     285,   290,   295,   296,   297,   298,   299,   304,   313,   314,
-     315,   316,   317,   318,   319,   320,   320,   321,   322,   346,
-     347,   348,   349,   353,   354,   363,   364,   365,   369,   376,
-     383,   387,   391,   398,   413,   414,   418,   430,   430,   435,
-     435,   440,   451,   466,   467,   468,   469,   473,   474,   479,
-     481,   486,   487,   492,   494,   499,   500,   504,   505,   506,
-     507,   512,   517,   522,   528,   534,   545,   546,   555,   556,
-     562,   563,   564,   571,   571,   576,   577,   578,   583,   585,
-     587,   589,   591,   593,   598,   600,   611,   612,   617,   618,
-     619,   628,   648,   650,   659,   664,   665,   670,   677,   679
+       0,   251,   251,   259,   261,   265,   266,   276,   280,   285,
+     286,   291,   296,   297,   298,   299,   300,   305,   314,   315,
+     316,   317,   318,   319,   320,   321,   321,   322,   323,   347,
+     348,   349,   350,   354,   355,   364,   365,   366,   370,   377,
+     384,   388,   392,   399,   414,   415,   419,   431,   431,   436,
+     436,   441,   452,   467,   468,   469,   470,   474,   475,   480,
+     482,   487,   488,   493,   495,   500,   501,   505,   506,   507,
+     508,   513,   518,   523,   529,   535,   546,   547,   556,   557,
+     563,   564,   565,   572,   572,   577,   578,   579,   584,   586,
+     588,   590,   592,   594,   596,   601,   603,   613,   614,   619,
+     620,   621,   630,   650,   652,   661,   666,   667,   672,   679,
+     681
 };
 #endif
 
@@ -672,10 +677,10 @@ static const char *const yytname[] =
   "\"%name-prefix\"", "\"%no-default-prec\"", "\"%no-lines\"",
   "\"%nondeterministic-parser\"", "\"%output\"", "\"%require\"",
   "\"%skeleton\"", "\"%start\"", "\"%token-table\"", "\"%verbose\"",
-  "\"%yacc\"", "\"{...}\"", "\"[identifier]\"", "\"char\"", "\"epilogue\"",
-  "\"=\"", "\"identifier\"", "\"identifier:\"", "\"%%\"", "\"|\"",
-  "\"%{...%}\"", "\";\"", "\"<tag>\"", "\"<*>\"", "\"<>\"", "\"%param\"",
-  "\"%union\"", "$accept", "input", "prologue_declarations",
+  "\"%yacc\"", "\"{...}\"", "\"%?{...}\"", "\"[identifier]\"", "\"char\"",
+  "\"epilogue\"", "\"=\"", "\"identifier\"", "\"identifier:\"", "\"%%\"",
+  "\"|\"", "\"%{...%}\"", "\";\"", "\"<tag>\"", "\"<*>\"", "\"<>\"",
+  "\"%param\"", "\"%union\"", "$accept", "input", "prologue_declarations",
   "prologue_declaration", "$@1", "params", "grammar_declaration",
   "union_name", "symbol_declaration", "$@2", "$@3",
   "precedence_declaration", "precedence_declarator", "tag.opt",
@@ -697,16 +702,16 @@ static const yytype_uint16 yytoknum[] =
      275,   276,   277,   278,   279,   280,   281,   282,   283,   284,
      285,   286,   287,   288,   289,   290,   291,   292,   293,   294,
      295,   296,   297,   298,   299,   300,   301,   302,   303,   304,
-     305,   306,   307,   308,   309,   310
+     305,   306,   307,   308,   309,   310,   311
 };
 # endif
 
-#define YYPACT_NINF -137
+#define YYPACT_NINF -50
 
 #define yypact_value_is_default(yystate) \
-  ((yystate) == (-137))
+  ((yystate) == (-50))
 
-#define YYTABLE_NINF -109
+#define YYTABLE_NINF -110
 
 #define yytable_value_is_error(yytable_value) \
   YYID (0)
@@ -715,21 +720,21 @@ static const yytype_uint16 yytoknum[] =
      STATE-NUM.    */
 static const yytype_int16 yypact[] =
 {
-    -137,    20,   106,  -137,  -137,  -137,    19,    -1,    36,  -137,
-    -137,  -137,  -137,    13,  -137,     0,    79,  -137,    89,    90,
-    -137,    41,  -137,    55,    93,    46,  -137,  -137,  -137,    47,
-      94,    95,    35,  -137,  -137,  -137,    16,  -137,  -137,  -137,
-      54,  -137,  -137,  -137,  -137,    49,    33,    33,    35,    12,
-      12,  -137,    61,  -137,  -137,  -137,    29,  -137,  -137,  -137,
-    -137,   100,  -137,  -137,  -137,   101,  -137,   103,  -137,  -137,
-    -137,  -137,  -137,  -137,  -137,  -137,    57,  -137,    58,     1,
-    -137,  -137,    68,    80,  -137,    61,  -137,    35,  -137,  -137,
-      33,    56,    33,    35,  -137,  -137,  -137,  -137,    12,  -137,
-    -137,    12,  -137,  -137,  -137,  -137,  -137,  -137,  -137,  -137,
-    -137,    78,  -137,  -137,  -137,  -137,  -137,    82,  -137,    35,
-    -137,   142,  -137,   144,  -137,  -137,  -137,  -137,  -137,  -137,
-    -137,  -137,  -137,    18,    27,  -137,  -137,    35,   145,    97,
-      68,    68,    27,  -137,  -137,  -137,  -137,  -137
+     -50,     4,   107,   -50,   -50,   -50,   -20,     3,    11,   -50,
+     -50,   -50,   -50,    14,   -50,    12,    68,   -50,    72,    73,
+     -50,     0,   -50,    45,    86,     2,   -50,   -50,   -50,    17,
+      95,    96,    32,   -50,   -50,   -50,    16,   -50,   -50,   -50,
+      54,   -50,   -50,   -50,   -50,    49,    -2,    -2,    32,    27,
+      27,   -50,    63,   -50,   -50,   -50,    37,   -50,   -50,   -50,
+     -50,   101,   -50,   -50,   -50,   103,   -50,   104,   -50,   -50,
+     -50,   -50,   -50,   -50,   -50,   -50,    57,   -50,    58,     1,
+     -50,   -50,    79,    70,   -50,    63,   -50,    32,   -50,   -50,
+      -2,    64,    -2,    32,   -50,   -50,   -50,   -50,    27,   -50,
+     -50,    27,   -50,   -50,   -50,   -50,   -50,   -50,   -50,   -50,
+     -50,    78,   -50,   -50,   -50,   -50,   -50,    83,   -50,    32,
+     -50,   143,   -50,   145,   -50,   -50,   -50,   -50,   -50,   -50,
+     -50,   -50,   -50,    43,    50,   -50,   -50,    32,   146,    97,
+      79,   -50,    79,    50,   -50,   -50,   -50,   -50,   -50
 };
 
   /* YYDEFACT[S] -- default reduction number in state S.  Performed when
@@ -742,25 +747,25 @@ static const yytype_uint8 yydefact[] =
        7,     0,    16,     0,     0,     0,    41,    21,    22,     0,
        0,     0,     0,    29,    30,    31,     0,     6,    32,    25,
       44,     4,     5,    36,    35,    57,     0,     0,     0,     0,
-       0,   101,     0,    42,    97,    96,    98,    10,    12,    13,
+       0,   102,     0,    42,    98,    97,    99,    10,    12,    13,
       14,     0,    17,    18,    19,     0,    23,     0,    27,    28,
-     107,   103,   102,   105,    37,   106,     0,   104,     0,     0,
-      78,    80,    94,     0,    45,     0,    58,     0,    71,    76,
+     108,   104,   103,   106,    37,   107,     0,   105,     0,     0,
+      78,    80,    95,     0,    45,     0,    58,     0,    71,    76,
       50,    72,    48,    51,    63,    68,    69,    70,    38,    65,
-      67,    39,    43,   100,    99,     8,    15,    20,    24,    82,
-      81,     0,    79,     2,    95,    83,    34,    26,    46,    52,
-      59,    61,    77,    73,    74,    64,    66,   109,    88,    33,
+      67,    39,    43,   101,   100,     8,    15,    20,    24,    82,
+      81,     0,    79,     2,    96,    83,    34,    26,    46,    52,
+      59,    61,    77,    73,    74,    64,    66,   110,    88,    33,
       60,    62,    75,    84,    85,    88,    87,     0,     0,     0,
-      94,    94,    86,    91,    92,    93,    90,    89
+      95,    91,    95,    86,    92,    93,    94,    90,    89
 };
 
   /* YYPGOTO[NTERM-NUM].    */
 static const yytype_int16 yypgoto[] =
 {
-    -137,  -137,  -137,  -137,  -137,  -137,   148,  -137,  -137,  -137,
-    -137,  -137,  -137,  -137,  -137,    32,  -137,   102,    -9,    -4,
-     107,  -137,    83,  -137,  -137,  -137,    22,  -136,  -137,  -137,
-     -12,   -11,  -137,   -32,   -40,  -137
+     -50,   -50,   -50,   -50,   -50,   -50,   149,   -50,   -50,   -50,
+     -50,   -50,   -50,   -50,   -50,    33,   -50,   106,   -42,    -4,
+     112,   -50,    74,   -50,   -50,   -50,    19,   -45,   -50,   -50,
+     -16,    -8,   -50,   -32,   -49,   -50
 };
 
   /* YYDEFGOTO[NTERM-NUM].    */
@@ -777,81 +782,82 @@ static const yytype_int16 yydefgoto[] =
      number is the opposite.  If YYTABLE_NINF, syntax error.    */
 static const yytype_int16 yytable[] =
 {
-      74,  -108,    76,    54,   146,   147,     4,     5,     6,     7,
-       8,     9,    10,    11,    12,    70,    94,    76,    13,    14,
-       3,     4,     5,     6,     7,     8,     9,    10,    11,    12,
-      70,    26,   103,    13,    14,    91,    91,    32,    70,    49,
-     102,   137,   138,   139,    60,    55,    26,    77,   111,    64,
-      66,   124,    32,    51,    71,   121,    40,    72,    52,    70,
-     123,   125,    77,    95,    96,    97,   135,   140,   136,    71,
-      48,    40,    72,   118,   104,    71,    50,    71,    72,    91,
-      72,    91,    57,   132,    88,    61,   122,   121,   122,   126,
-      65,    67,   126,    58,    59,    62,    63,    68,    69,    84,
-      86,    51,   141,   106,   107,   143,   108,   109,   110,   114,
-     141,     4,     5,     6,     7,     8,     9,    10,    11,    12,
-     116,   127,   129,    13,    14,    15,    16,    17,    18,    19,
-      20,    21,    22,    23,    24,    25,    26,    27,    28,    29,
-      30,    31,    32,    33,    34,    35,   131,    70,   145,   144,
-      42,   130,   101,    36,    92,    37,    38,   142,     0,     0,
-      39,    40,   112
+      74,  -109,    76,    60,     3,    64,     4,     5,     6,     7,
+       8,     9,    10,    11,    12,    54,    94,    76,    13,    14,
+      66,     4,     5,     6,     7,     8,     9,    10,    11,    12,
+      70,    26,    48,    13,    14,    70,   102,    32,    91,    91,
+     103,    71,   124,    49,    72,    61,    26,    65,    77,   111,
+      88,    50,    32,    70,    51,   121,   126,    40,    55,   126,
+      52,   125,    67,    77,   137,   138,   139,    70,   123,   118,
+      71,    57,    40,    72,   132,    71,    58,    59,    72,    95,
+      96,    97,    91,   104,    91,    62,   122,   121,   122,    63,
+     140,   141,   135,    71,   136,   147,    72,   148,    68,    69,
+      84,    86,   142,    51,   106,   144,   107,   108,   109,   110,
+     116,   142,     4,     5,     6,     7,     8,     9,    10,    11,
+      12,   114,   127,   129,    13,    14,    15,    16,    17,    18,
+      19,    20,    21,    22,    23,    24,    25,    26,    27,    28,
+      29,    30,    31,    32,    33,    34,    35,   131,    70,   146,
+     145,    42,   130,   112,   143,    36,   101,    37,    38,    92,
+       0,     0,    39,    40
 };
 
 static const yytype_int16 yycheck[] =
 {
-      32,     0,     1,     3,   140,   141,     5,     6,     7,     8,
+      32,     0,     1,     3,     0,     3,     5,     6,     7,     8,
        9,    10,    11,    12,    13,     3,    48,     1,    17,    18,
-       0,     5,     6,     7,     8,     9,    10,    11,    12,    13,
-       3,    30,     3,    17,    18,    46,    47,    36,     3,    40,
-      52,    14,    15,    16,     3,    45,    30,    46,    47,     3,
-       3,    91,    36,    40,    42,    87,    55,    45,    45,     3,
-       4,    93,    46,    51,    52,    53,    48,    40,    50,    42,
-      51,    55,    45,    85,    45,    42,    40,    42,    45,    90,
-      45,    92,     3,   123,    51,    44,    90,   119,    92,    98,
-      44,    44,   101,     4,     4,    40,     3,     3,     3,    45,
-      51,    40,   134,     3,     3,   137,     3,    50,    50,    41,
-     142,     5,     6,     7,     8,     9,    10,    11,    12,    13,
-      40,    43,    40,    17,    18,    19,    20,    21,    22,    23,
-      24,    25,    26,    27,    28,    29,    30,    31,    32,    33,
-      34,    35,    36,    37,    38,    39,     4,     3,    51,     4,
-       2,   119,    50,    47,    47,    49,    50,   135,    -1,    -1,
-      54,    55,    79
+       3,     5,     6,     7,     8,     9,    10,    11,    12,    13,
+       3,    30,    52,    17,    18,     3,    52,    36,    46,    47,
+       3,    43,    91,    40,    46,    45,    30,    45,    47,    48,
+      52,    40,    36,     3,    40,    87,    98,    56,    46,   101,
+      46,    93,    45,    47,    14,    15,    16,     3,     4,    85,
+      43,     3,    56,    46,   123,    43,     4,     4,    46,    52,
+      53,    54,    90,    46,    92,    40,    90,   119,    92,     3,
+      40,    41,    49,    43,    51,   140,    46,   142,     3,     3,
+      46,    52,   134,    40,     3,   137,     3,     3,    51,    51,
+      40,   143,     5,     6,     7,     8,     9,    10,    11,    12,
+      13,    42,    44,    40,    17,    18,    19,    20,    21,    22,
+      23,    24,    25,    26,    27,    28,    29,    30,    31,    32,
+      33,    34,    35,    36,    37,    38,    39,     4,     3,    52,
+       4,     2,   119,    79,   135,    48,    50,    50,    51,    47,
+      -1,    -1,    55,    56
 };
 
   /* STOS_[STATE-NUM] -- The (internal number of the) accessing
      symbol of state STATE-NUM.    */
 static const yytype_uint8 yystos[] =
 {
-       0,    57,    58,     0,     5,     6,     7,     8,     9,    10,
+       0,    58,    59,     0,     5,     6,     7,     8,     9,    10,
       11,    12,    13,    17,    18,    19,    20,    21,    22,    23,
       24,    25,    26,    27,    28,    29,    30,    31,    32,    33,
-      34,    35,    36,    37,    38,    39,    47,    49,    50,    54,
-      55,    59,    62,    64,    67,    68,    66,    65,    51,    40,
-      40,    40,    45,    86,     3,    45,    84,     3,     4,     4,
-       3,    44,    40,     3,     3,    44,     3,    44,     3,     3,
-       3,    42,    45,    87,    89,    90,     1,    46,    62,    77,
-      78,    79,    88,    60,    45,    63,    51,    69,    51,    75,
-      76,    87,    76,    72,    89,    51,    52,    53,    73,    74,
-      89,    73,    86,     3,    45,    85,     3,     3,     3,    50,
-      50,    47,    78,    91,    41,    83,    40,    61,    86,    70,
-      71,    89,    75,     4,    90,    89,    74,    43,    80,    40,
-      71,     4,    90,    81,    82,    48,    50,    14,    15,    16,
-      40,    89,    82,    89,     4,    51,    83,    83
+      34,    35,    36,    37,    38,    39,    48,    50,    51,    55,
+      56,    60,    63,    65,    68,    69,    67,    66,    52,    40,
+      40,    40,    46,    87,     3,    46,    85,     3,     4,     4,
+       3,    45,    40,     3,     3,    45,     3,    45,     3,     3,
+       3,    43,    46,    88,    90,    91,     1,    47,    63,    78,
+      79,    80,    89,    61,    46,    64,    52,    70,    52,    76,
+      77,    88,    77,    73,    90,    52,    53,    54,    74,    75,
+      90,    74,    87,     3,    46,    86,     3,     3,     3,    51,
+      51,    48,    79,    92,    42,    84,    40,    62,    87,    71,
+      72,    90,    76,     4,    91,    90,    75,    44,    81,    40,
+      72,     4,    91,    82,    83,    49,    51,    14,    15,    16,
+      40,    41,    90,    83,    90,     4,    52,    84,    84
 };
 
   /* YYR1[YYN] -- Symbol number of symbol that rule YYN derives.    */
 static const yytype_uint8 yyr1[] =
 {
-       0,    56,    57,    58,    58,    59,    59,    59,    59,    59,
-      59,    59,    59,    59,    59,    59,    59,    59,    59,    59,
-      59,    59,    59,    59,    59,    60,    59,    59,    59,    59,
-      59,    59,    59,    61,    61,    62,    62,    62,    62,    62,
-      62,    62,    62,    62,    63,    63,    62,    65,    64,    66,
-      64,    64,    67,    68,    68,    68,    68,    69,    69,    70,
-      70,    71,    71,    72,    72,    73,    73,    74,    74,    74,
-      74,    75,    75,    75,    75,    75,    76,    76,    77,    77,
-      78,    78,    78,    80,    79,    81,    81,    81,    82,    82,
-      82,    82,    82,    82,    83,    83,    84,    84,    85,    85,
-      85,    86,    87,    87,    88,    89,    89,    90,    91,    91
+       0,    57,    58,    59,    59,    60,    60,    60,    60,    60,
+      60,    60,    60,    60,    60,    60,    60,    60,    60,    60,
+      60,    60,    60,    60,    60,    61,    60,    60,    60,    60,
+      60,    60,    60,    62,    62,    63,    63,    63,    63,    63,
+      63,    63,    63,    63,    64,    64,    63,    66,    65,    67,
+      65,    65,    68,    69,    69,    69,    69,    70,    70,    71,
+      71,    72,    72,    73,    73,    74,    74,    75,    75,    75,
+      75,    76,    76,    76,    76,    76,    77,    77,    78,    78,
+      79,    79,    79,    81,    80,    82,    82,    82,    83,    83,
+      83,    83,    83,    83,    83,    84,    84,    85,    85,    86,
+      86,    86,    87,    88,    88,    89,    90,    90,    91,    92,
+      92
 };
 
   /* YYR2[YYN] -- Number of symbols composing right hand side of rule YYN.    */
@@ -866,8 +872,9 @@ static const yytype_uint8 yyr2[] =
        2,     1,     2,     1,     2,     1,     2,     1,     1,     1,
        1,     1,     1,     2,     2,     3,     1,     2,     1,     2,
        1,     2,     2,     0,     4,     1,     3,     2,     0,     3,
-       3,     3,     3,     3,     0,     1,     1,     1,     0,     1,
-       1,     1,     1,     1,     1,     1,     1,     1,     0,     2
+       3,     2,     3,     3,     3,     0,     1,     1,     1,     0,
+       1,     1,     1,     1,     1,     1,     1,     1,     1,     0,
+       2
 };
 
 
@@ -1027,106 +1034,106 @@ yy_symbol_value_print (yyoutput, yytype, yyvaluep, yylocationp)
             case 3: // "string"
 
 /* Line 729 of yacc.c  */
-#line 174 "src/parse-gram.y"
+#line 175 "src/parse-gram.y"
         { fputs (quotearg_style (c_quoting_style, ((*yyvaluep).chars)), stderr); }
 /* Line 729 of yacc.c  */
-#line 1034 "src/parse-gram.c"
+#line 1041 "src/parse-gram.c"
         break;
 
             case 4: // "integer"
 
 /* Line 729 of yacc.c  */
-#line 187 "src/parse-gram.y"
+#line 188 "src/parse-gram.y"
         { fprintf (stderr, "%d", ((*yyvaluep).integer)); }
 /* Line 729 of yacc.c  */
-#line 1043 "src/parse-gram.c"
+#line 1050 "src/parse-gram.c"
         break;
 
             case 24: // "%<flag>"
 
 /* Line 729 of yacc.c  */
-#line 183 "src/parse-gram.y"
+#line 184 "src/parse-gram.y"
         { fprintf (stderr, "%%%s", ((*yyvaluep).uniqstr)); }
 /* Line 729 of yacc.c  */
-#line 1052 "src/parse-gram.c"
+#line 1059 "src/parse-gram.c"
         break;
 
             case 40: // "{...}"
 
 /* Line 729 of yacc.c  */
-#line 176 "src/parse-gram.y"
+#line 177 "src/parse-gram.y"
         { fprintf (stderr, "{\n%s\n}", ((*yyvaluep).code)); }
 /* Line 729 of yacc.c  */
-#line 1061 "src/parse-gram.c"
+#line 1068 "src/parse-gram.c"
         break;
 
-            case 41: // "[identifier]"
+            case 42: // "[identifier]"
 
 /* Line 729 of yacc.c  */
-#line 181 "src/parse-gram.y"
+#line 182 "src/parse-gram.y"
         { fprintf (stderr, "[%s]", ((*yyvaluep).uniqstr)); }
 /* Line 729 of yacc.c  */
-#line 1070 "src/parse-gram.c"
+#line 1077 "src/parse-gram.c"
         break;
 
-            case 42: // "char"
+            case 43: // "char"
 
 /* Line 729 of yacc.c  */
-#line 168 "src/parse-gram.y"
+#line 169 "src/parse-gram.y"
         { fputs (char_name (((*yyvaluep).character)), stderr); }
 /* Line 729 of yacc.c  */
-#line 1079 "src/parse-gram.c"
+#line 1086 "src/parse-gram.c"
         break;
 
-            case 43: // "epilogue"
+            case 44: // "epilogue"
 
 /* Line 729 of yacc.c  */
-#line 176 "src/parse-gram.y"
+#line 177 "src/parse-gram.y"
         { fprintf (stderr, "{\n%s\n}", ((*yyvaluep).chars)); }
 /* Line 729 of yacc.c  */
-#line 1088 "src/parse-gram.c"
+#line 1095 "src/parse-gram.c"
         break;
 
-            case 45: // "identifier"
+            case 46: // "identifier"
 
 /* Line 729 of yacc.c  */
-#line 180 "src/parse-gram.y"
+#line 181 "src/parse-gram.y"
         { fputs (((*yyvaluep).uniqstr), stderr); }
 /* Line 729 of yacc.c  */
-#line 1097 "src/parse-gram.c"
+#line 1104 "src/parse-gram.c"
         break;
 
-            case 46: // "identifier:"
+            case 47: // "identifier:"
 
 /* Line 729 of yacc.c  */
-#line 182 "src/parse-gram.y"
+#line 183 "src/parse-gram.y"
         { fprintf (stderr, "%s:", ((*yyvaluep).uniqstr)); }
 /* Line 729 of yacc.c  */
-#line 1106 "src/parse-gram.c"
+#line 1113 "src/parse-gram.c"
         break;
 
-            case 49: // "%{...%}"
+            case 50: // "%{...%}"
 
 /* Line 729 of yacc.c  */
-#line 176 "src/parse-gram.y"
+#line 177 "src/parse-gram.y"
         { fprintf (stderr, "{\n%s\n}", ((*yyvaluep).chars)); }
 /* Line 729 of yacc.c  */
-#line 1115 "src/parse-gram.c"
+#line 1122 "src/parse-gram.c"
         break;
 
-            case 51: // "<tag>"
+            case 52: // "<tag>"
 
 /* Line 729 of yacc.c  */
-#line 184 "src/parse-gram.y"
+#line 185 "src/parse-gram.y"
         { fprintf (stderr, "<%s>", ((*yyvaluep).uniqstr)); }
 /* Line 729 of yacc.c  */
-#line 1124 "src/parse-gram.c"
+#line 1131 "src/parse-gram.c"
         break;
 
-            case 54: // "%param"
+            case 55: // "%param"
 
 /* Line 729 of yacc.c  */
-#line 230 "src/parse-gram.y"
+#line 231 "src/parse-gram.y"
         {
   switch (((*yyvaluep).param))
     {
@@ -1140,79 +1147,79 @@ yy_symbol_value_print (yyoutput, yytype, yyvaluep, yylocationp)
     }
 }
 /* Line 729 of yacc.c  */
-#line 1144 "src/parse-gram.c"
+#line 1151 "src/parse-gram.c"
         break;
 
-            case 71: // symbol.prec
+            case 72: // symbol.prec
 
 /* Line 729 of yacc.c  */
-#line 190 "src/parse-gram.y"
+#line 191 "src/parse-gram.y"
         { fprintf (stderr, "%s", ((*yyvaluep).symbol)->tag); }
 /* Line 729 of yacc.c  */
-#line 1153 "src/parse-gram.c"
+#line 1160 "src/parse-gram.c"
         break;
 
-            case 84: // variable
+            case 85: // variable
 
 /* Line 729 of yacc.c  */
-#line 180 "src/parse-gram.y"
+#line 181 "src/parse-gram.y"
         { fputs (((*yyvaluep).uniqstr), stderr); }
 /* Line 729 of yacc.c  */
-#line 1162 "src/parse-gram.c"
+#line 1169 "src/parse-gram.c"
         break;
 
-            case 85: // content.opt
+            case 86: // content.opt
 
 /* Line 729 of yacc.c  */
-#line 176 "src/parse-gram.y"
+#line 177 "src/parse-gram.y"
         { fprintf (stderr, "{\n%s\n}", ((*yyvaluep).chars)); }
 /* Line 729 of yacc.c  */
-#line 1171 "src/parse-gram.c"
+#line 1178 "src/parse-gram.c"
         break;
 
-            case 86: // braceless
+            case 87: // braceless
 
 /* Line 729 of yacc.c  */
-#line 176 "src/parse-gram.y"
+#line 177 "src/parse-gram.y"
         { fprintf (stderr, "{\n%s\n}", ((*yyvaluep).chars)); }
 /* Line 729 of yacc.c  */
-#line 1180 "src/parse-gram.c"
+#line 1187 "src/parse-gram.c"
         break;
 
-            case 87: // id
+            case 88: // id
 
 /* Line 729 of yacc.c  */
-#line 190 "src/parse-gram.y"
+#line 191 "src/parse-gram.y"
         { fprintf (stderr, "%s", ((*yyvaluep).symbol)->tag); }
 /* Line 729 of yacc.c  */
-#line 1189 "src/parse-gram.c"
+#line 1196 "src/parse-gram.c"
         break;
 
-            case 88: // id_colon
+            case 89: // id_colon
 
 /* Line 729 of yacc.c  */
-#line 191 "src/parse-gram.y"
+#line 192 "src/parse-gram.y"
         { fprintf (stderr, "%s:", ((*yyvaluep).symbol)->tag); }
 /* Line 729 of yacc.c  */
-#line 1198 "src/parse-gram.c"
+#line 1205 "src/parse-gram.c"
         break;
 
-            case 89: // symbol
+            case 90: // symbol
 
 /* Line 729 of yacc.c  */
-#line 190 "src/parse-gram.y"
+#line 191 "src/parse-gram.y"
         { fprintf (stderr, "%s", ((*yyvaluep).symbol)->tag); }
 /* Line 729 of yacc.c  */
-#line 1207 "src/parse-gram.c"
+#line 1214 "src/parse-gram.c"
         break;
 
-            case 90: // string_as_id
+            case 91: // string_as_id
 
 /* Line 729 of yacc.c  */
-#line 190 "src/parse-gram.y"
+#line 191 "src/parse-gram.y"
         { fprintf (stderr, "%s", ((*yyvaluep).symbol)->tag); }
 /* Line 729 of yacc.c  */
-#line 1216 "src/parse-gram.c"
+#line 1223 "src/parse-gram.c"
         break;
 
       default:
@@ -1743,7 +1750,7 @@ YYLTYPE yylloc;
 
 /* User initialization code.  */
 /* Line 1214 of yacc.c  */
-#line 79 "src/parse-gram.y"
+#line 80 "src/parse-gram.y"
 {
   /* Bison's grammar can initial empty locations, hence a default
      location is needed. */
@@ -1751,7 +1758,7 @@ YYLTYPE yylloc;
   boundary_set (&yylloc.end, current_file, 1, 1);
 }
 /* Line 1214 of yacc.c  */
-#line 1755 "src/parse-gram.c"
+#line 1762 "src/parse-gram.c"
   yylsp[0] = yylloc;
 
   goto yysetstate;
@@ -1938,7 +1945,7 @@ yyreduce:
     {
         case 6:
 /* Line 1427 of yacc.c  */
-#line 266 "src/parse-gram.y"
+#line 267 "src/parse-gram.y"
     {
       code_props plain_code;
       code_props_plain_init (&plain_code, (yyvsp[0].chars), (yylsp[0]));
@@ -1949,106 +1956,106 @@ yyreduce:
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 1953 "src/parse-gram.c"
+#line 1960 "src/parse-gram.c"
     break;
 
   case 7:
 /* Line 1427 of yacc.c  */
-#line 276 "src/parse-gram.y"
+#line 277 "src/parse-gram.y"
     {
       muscle_percent_define_ensure ((yyvsp[0].uniqstr), (yylsp[0]), true);
     }
 /* Line 1427 of yacc.c  */
-#line 1963 "src/parse-gram.c"
+#line 1970 "src/parse-gram.c"
     break;
 
   case 8:
 /* Line 1427 of yacc.c  */
-#line 280 "src/parse-gram.y"
+#line 281 "src/parse-gram.y"
     {
       muscle_percent_define_insert ((yyvsp[-1].uniqstr), (yylsp[-1]), (yyvsp[0].chars),
                                     MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE);
     }
 /* Line 1427 of yacc.c  */
-#line 1974 "src/parse-gram.c"
+#line 1981 "src/parse-gram.c"
     break;
 
   case 9:
 /* Line 1427 of yacc.c  */
-#line 284 "src/parse-gram.y"
+#line 285 "src/parse-gram.y"
     { defines_flag = true; }
 /* Line 1427 of yacc.c  */
-#line 1982 "src/parse-gram.c"
+#line 1989 "src/parse-gram.c"
     break;
 
   case 10:
 /* Line 1427 of yacc.c  */
-#line 286 "src/parse-gram.y"
+#line 287 "src/parse-gram.y"
     {
       defines_flag = true;
       spec_defines_file = xstrdup ((yyvsp[0].chars));
     }
 /* Line 1427 of yacc.c  */
-#line 1993 "src/parse-gram.c"
+#line 2000 "src/parse-gram.c"
     break;
 
   case 11:
 /* Line 1427 of yacc.c  */
-#line 291 "src/parse-gram.y"
+#line 292 "src/parse-gram.y"
     {
       muscle_percent_define_insert ("parse.error", (yylsp[0]), "verbose",
                                     MUSCLE_PERCENT_DEFINE_GRAMMAR_FILE);
     }
 /* Line 1427 of yacc.c  */
-#line 2004 "src/parse-gram.c"
+#line 2011 "src/parse-gram.c"
     break;
 
   case 12:
 /* Line 1427 of yacc.c  */
-#line 295 "src/parse-gram.y"
+#line 296 "src/parse-gram.y"
     { expected_sr_conflicts = (yyvsp[0].integer); }
 /* Line 1427 of yacc.c  */
-#line 2012 "src/parse-gram.c"
+#line 2019 "src/parse-gram.c"
     break;
 
   case 13:
 /* Line 1427 of yacc.c  */
-#line 296 "src/parse-gram.y"
+#line 297 "src/parse-gram.y"
     { expected_rr_conflicts = (yyvsp[0].integer); }
 /* Line 1427 of yacc.c  */
-#line 2020 "src/parse-gram.c"
+#line 2027 "src/parse-gram.c"
     break;
 
   case 14:
 /* Line 1427 of yacc.c  */
-#line 297 "src/parse-gram.y"
+#line 298 "src/parse-gram.y"
     { spec_file_prefix = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2028 "src/parse-gram.c"
+#line 2035 "src/parse-gram.c"
     break;
 
   case 15:
 /* Line 1427 of yacc.c  */
-#line 298 "src/parse-gram.y"
+#line 299 "src/parse-gram.y"
     { spec_file_prefix = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2036 "src/parse-gram.c"
+#line 2043 "src/parse-gram.c"
     break;
 
   case 16:
 /* Line 1427 of yacc.c  */
-#line 300 "src/parse-gram.y"
+#line 301 "src/parse-gram.y"
     {
       nondeterministic_parser = true;
       glr_parser = true;
     }
 /* Line 1427 of yacc.c  */
-#line 2047 "src/parse-gram.c"
+#line 2054 "src/parse-gram.c"
     break;
 
   case 17:
 /* Line 1427 of yacc.c  */
-#line 305 "src/parse-gram.y"
+#line 306 "src/parse-gram.y"
     {
       code_props action;
       code_props_symbol_action_init (&action, (yyvsp[0].code), (yylsp[0]));
@@ -2058,92 +2065,92 @@ yyreduce:
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 2062 "src/parse-gram.c"
+#line 2069 "src/parse-gram.c"
     break;
 
   case 18:
 /* Line 1427 of yacc.c  */
-#line 313 "src/parse-gram.y"
+#line 314 "src/parse-gram.y"
     { language_argmatch ((yyvsp[0].chars), grammar_prio, (yylsp[-1])); }
 /* Line 1427 of yacc.c  */
-#line 2070 "src/parse-gram.c"
+#line 2077 "src/parse-gram.c"
     break;
 
   case 19:
 /* Line 1427 of yacc.c  */
-#line 314 "src/parse-gram.y"
+#line 315 "src/parse-gram.y"
     { spec_name_prefix = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2078 "src/parse-gram.c"
+#line 2085 "src/parse-gram.c"
     break;
 
   case 20:
 /* Line 1427 of yacc.c  */
-#line 315 "src/parse-gram.y"
+#line 316 "src/parse-gram.y"
     { spec_name_prefix = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2086 "src/parse-gram.c"
+#line 2093 "src/parse-gram.c"
     break;
 
   case 21:
 /* Line 1427 of yacc.c  */
-#line 316 "src/parse-gram.y"
+#line 317 "src/parse-gram.y"
     { no_lines_flag = true; }
 /* Line 1427 of yacc.c  */
-#line 2094 "src/parse-gram.c"
+#line 2101 "src/parse-gram.c"
     break;
 
   case 22:
 /* Line 1427 of yacc.c  */
-#line 317 "src/parse-gram.y"
+#line 318 "src/parse-gram.y"
     { nondeterministic_parser = true; }
 /* Line 1427 of yacc.c  */
-#line 2102 "src/parse-gram.c"
+#line 2109 "src/parse-gram.c"
     break;
 
   case 23:
 /* Line 1427 of yacc.c  */
-#line 318 "src/parse-gram.y"
+#line 319 "src/parse-gram.y"
     { spec_outfile = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2110 "src/parse-gram.c"
+#line 2117 "src/parse-gram.c"
     break;
 
   case 24:
 /* Line 1427 of yacc.c  */
-#line 319 "src/parse-gram.y"
+#line 320 "src/parse-gram.y"
     { spec_outfile = (yyvsp[0].chars); }
 /* Line 1427 of yacc.c  */
-#line 2118 "src/parse-gram.c"
+#line 2125 "src/parse-gram.c"
     break;
 
   case 25:
 /* Line 1427 of yacc.c  */
-#line 320 "src/parse-gram.y"
+#line 321 "src/parse-gram.y"
     { current_param = (yyvsp[0].param); }
 /* Line 1427 of yacc.c  */
-#line 2126 "src/parse-gram.c"
+#line 2133 "src/parse-gram.c"
     break;
 
   case 26:
 /* Line 1427 of yacc.c  */
-#line 320 "src/parse-gram.y"
+#line 321 "src/parse-gram.y"
     { current_param = param_none; }
 /* Line 1427 of yacc.c  */
-#line 2134 "src/parse-gram.c"
+#line 2141 "src/parse-gram.c"
     break;
 
   case 27:
 /* Line 1427 of yacc.c  */
-#line 321 "src/parse-gram.y"
+#line 322 "src/parse-gram.y"
     { version_check (&(yylsp[0]), (yyvsp[0].chars)); }
 /* Line 1427 of yacc.c  */
-#line 2142 "src/parse-gram.c"
+#line 2149 "src/parse-gram.c"
     break;
 
   case 28:
 /* Line 1427 of yacc.c  */
-#line 323 "src/parse-gram.y"
+#line 324 "src/parse-gram.y"
     {
       char const *skeleton_user = (yyvsp[0].chars);
       if (strchr (skeleton_user, '/'))
@@ -2168,62 +2175,62 @@ yyreduce:
       skeleton_arg (skeleton_user, grammar_prio, (yylsp[-1]));
     }
 /* Line 1427 of yacc.c  */
-#line 2172 "src/parse-gram.c"
+#line 2179 "src/parse-gram.c"
     break;
 
   case 29:
 /* Line 1427 of yacc.c  */
-#line 346 "src/parse-gram.y"
+#line 347 "src/parse-gram.y"
     { token_table_flag = true; }
 /* Line 1427 of yacc.c  */
-#line 2180 "src/parse-gram.c"
+#line 2187 "src/parse-gram.c"
     break;
 
   case 30:
 /* Line 1427 of yacc.c  */
-#line 347 "src/parse-gram.y"
+#line 348 "src/parse-gram.y"
     { report_flag |= report_states; }
 /* Line 1427 of yacc.c  */
-#line 2188 "src/parse-gram.c"
+#line 2195 "src/parse-gram.c"
     break;
 
   case 31:
 /* Line 1427 of yacc.c  */
-#line 348 "src/parse-gram.y"
+#line 349 "src/parse-gram.y"
     { yacc_flag = true; }
 /* Line 1427 of yacc.c  */
-#line 2196 "src/parse-gram.c"
+#line 2203 "src/parse-gram.c"
     break;
 
   case 33:
 /* Line 1427 of yacc.c  */
-#line 353 "src/parse-gram.y"
+#line 354 "src/parse-gram.y"
     { add_param (current_param, (yyvsp[0].code), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2204 "src/parse-gram.c"
+#line 2211 "src/parse-gram.c"
     break;
 
   case 34:
 /* Line 1427 of yacc.c  */
-#line 354 "src/parse-gram.y"
+#line 355 "src/parse-gram.y"
     { add_param (current_param, (yyvsp[0].code), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2212 "src/parse-gram.c"
+#line 2219 "src/parse-gram.c"
     break;
 
   case 37:
 /* Line 1427 of yacc.c  */
-#line 366 "src/parse-gram.y"
+#line 367 "src/parse-gram.y"
     {
       grammar_start_symbol_set ((yyvsp[0].symbol), (yylsp[0]));
     }
 /* Line 1427 of yacc.c  */
-#line 2222 "src/parse-gram.c"
+#line 2229 "src/parse-gram.c"
     break;
 
   case 38:
 /* Line 1427 of yacc.c  */
-#line 370 "src/parse-gram.y"
+#line 371 "src/parse-gram.y"
     {
       symbol_list *list;
       for (list = (yyvsp[0].list); list; list = list->next)
@@ -2231,12 +2238,12 @@ yyreduce:
       symbol_list_free ((yyvsp[0].list));
     }
 /* Line 1427 of yacc.c  */
-#line 2235 "src/parse-gram.c"
+#line 2242 "src/parse-gram.c"
     break;
 
   case 39:
 /* Line 1427 of yacc.c  */
-#line 377 "src/parse-gram.y"
+#line 378 "src/parse-gram.y"
     {
       symbol_list *list;
       for (list = (yyvsp[0].list); list; list = list->next)
@@ -2244,32 +2251,32 @@ yyreduce:
       symbol_list_free ((yyvsp[0].list));
     }
 /* Line 1427 of yacc.c  */
-#line 2248 "src/parse-gram.c"
+#line 2255 "src/parse-gram.c"
     break;
 
   case 40:
 /* Line 1427 of yacc.c  */
-#line 384 "src/parse-gram.y"
+#line 385 "src/parse-gram.y"
     {
       default_prec = true;
     }
 /* Line 1427 of yacc.c  */
-#line 2258 "src/parse-gram.c"
+#line 2265 "src/parse-gram.c"
     break;
 
   case 41:
 /* Line 1427 of yacc.c  */
-#line 388 "src/parse-gram.y"
+#line 389 "src/parse-gram.y"
     {
       default_prec = false;
     }
 /* Line 1427 of yacc.c  */
-#line 2268 "src/parse-gram.c"
+#line 2275 "src/parse-gram.c"
     break;
 
   case 42:
 /* Line 1427 of yacc.c  */
-#line 392 "src/parse-gram.y"
+#line 393 "src/parse-gram.y"
     {
       /* Do not invoke muscle_percent_code_grow here since it invokes
          muscle_user_name_list_grow.  */
@@ -2277,89 +2284,89 @@ yyreduce:
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 2281 "src/parse-gram.c"
+#line 2288 "src/parse-gram.c"
     break;
 
   case 43:
 /* Line 1427 of yacc.c  */
-#line 399 "src/parse-gram.y"
+#line 400 "src/parse-gram.y"
     {
       muscle_percent_code_grow ((yyvsp[-1].uniqstr), (yylsp[-1]), (yyvsp[0].chars), (yylsp[0]));
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 2292 "src/parse-gram.c"
+#line 2299 "src/parse-gram.c"
     break;
 
   case 44:
 /* Line 1427 of yacc.c  */
-#line 413 "src/parse-gram.y"
+#line 414 "src/parse-gram.y"
     {}
 /* Line 1427 of yacc.c  */
-#line 2300 "src/parse-gram.c"
+#line 2307 "src/parse-gram.c"
     break;
 
   case 45:
 /* Line 1427 of yacc.c  */
-#line 414 "src/parse-gram.y"
+#line 415 "src/parse-gram.y"
     { muscle_code_grow ("union_name", (yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2308 "src/parse-gram.c"
+#line 2315 "src/parse-gram.c"
     break;
 
   case 46:
 /* Line 1427 of yacc.c  */
-#line 419 "src/parse-gram.y"
+#line 420 "src/parse-gram.y"
     {
       union_seen = true;
       muscle_code_grow ("stype", (yyvsp[0].chars), (yylsp[0]));
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 2320 "src/parse-gram.c"
+#line 2327 "src/parse-gram.c"
     break;
 
   case 47:
 /* Line 1427 of yacc.c  */
-#line 430 "src/parse-gram.y"
+#line 431 "src/parse-gram.y"
     { current_class = nterm_sym; }
 /* Line 1427 of yacc.c  */
-#line 2328 "src/parse-gram.c"
+#line 2335 "src/parse-gram.c"
     break;
 
   case 48:
 /* Line 1427 of yacc.c  */
-#line 431 "src/parse-gram.y"
+#line 432 "src/parse-gram.y"
     {
       current_class = unknown_sym;
       current_type = NULL;
     }
 /* Line 1427 of yacc.c  */
-#line 2339 "src/parse-gram.c"
+#line 2346 "src/parse-gram.c"
     break;
 
   case 49:
 /* Line 1427 of yacc.c  */
-#line 435 "src/parse-gram.y"
+#line 436 "src/parse-gram.y"
     { current_class = token_sym; }
 /* Line 1427 of yacc.c  */
-#line 2347 "src/parse-gram.c"
+#line 2354 "src/parse-gram.c"
     break;
 
   case 50:
 /* Line 1427 of yacc.c  */
-#line 436 "src/parse-gram.y"
+#line 437 "src/parse-gram.y"
     {
       current_class = unknown_sym;
       current_type = NULL;
     }
 /* Line 1427 of yacc.c  */
-#line 2358 "src/parse-gram.c"
+#line 2365 "src/parse-gram.c"
     break;
 
   case 51:
 /* Line 1427 of yacc.c  */
-#line 441 "src/parse-gram.y"
+#line 442 "src/parse-gram.y"
     {
       symbol_list *list;
       tag_seen = true;
@@ -2368,12 +2375,12 @@ yyreduce:
       symbol_list_free ((yyvsp[0].list));
     }
 /* Line 1427 of yacc.c  */
-#line 2372 "src/parse-gram.c"
+#line 2379 "src/parse-gram.c"
     break;
 
   case 52:
 /* Line 1427 of yacc.c  */
-#line 452 "src/parse-gram.y"
+#line 453 "src/parse-gram.y"
     {
       symbol_list *list;
       ++current_prec;
@@ -2386,202 +2393,202 @@ yyreduce:
       current_type = NULL;
     }
 /* Line 1427 of yacc.c  */
-#line 2390 "src/parse-gram.c"
+#line 2397 "src/parse-gram.c"
     break;
 
   case 53:
 /* Line 1427 of yacc.c  */
-#line 466 "src/parse-gram.y"
+#line 467 "src/parse-gram.y"
     { (yyval.assoc) = left_assoc; }
 /* Line 1427 of yacc.c  */
-#line 2398 "src/parse-gram.c"
+#line 2405 "src/parse-gram.c"
     break;
 
   case 54:
 /* Line 1427 of yacc.c  */
-#line 467 "src/parse-gram.y"
+#line 468 "src/parse-gram.y"
     { (yyval.assoc) = right_assoc; }
 /* Line 1427 of yacc.c  */
-#line 2406 "src/parse-gram.c"
+#line 2413 "src/parse-gram.c"
     break;
 
   case 55:
 /* Line 1427 of yacc.c  */
-#line 468 "src/parse-gram.y"
+#line 469 "src/parse-gram.y"
     { (yyval.assoc) = non_assoc; }
 /* Line 1427 of yacc.c  */
-#line 2414 "src/parse-gram.c"
+#line 2421 "src/parse-gram.c"
     break;
 
   case 56:
 /* Line 1427 of yacc.c  */
-#line 469 "src/parse-gram.y"
+#line 470 "src/parse-gram.y"
     { (yyval.assoc) = precedence_assoc; }
 /* Line 1427 of yacc.c  */
-#line 2422 "src/parse-gram.c"
+#line 2429 "src/parse-gram.c"
     break;
 
   case 57:
 /* Line 1427 of yacc.c  */
-#line 473 "src/parse-gram.y"
+#line 474 "src/parse-gram.y"
     { current_type = NULL; }
 /* Line 1427 of yacc.c  */
-#line 2430 "src/parse-gram.c"
+#line 2437 "src/parse-gram.c"
     break;
 
   case 58:
 /* Line 1427 of yacc.c  */
-#line 474 "src/parse-gram.y"
+#line 475 "src/parse-gram.y"
     { current_type = (yyvsp[0].uniqstr); tag_seen = true; }
 /* Line 1427 of yacc.c  */
-#line 2438 "src/parse-gram.c"
+#line 2445 "src/parse-gram.c"
     break;
 
   case 59:
 /* Line 1427 of yacc.c  */
-#line 480 "src/parse-gram.y"
+#line 481 "src/parse-gram.y"
     { (yyval.list) = symbol_list_sym_new ((yyvsp[0].symbol), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2446 "src/parse-gram.c"
+#line 2453 "src/parse-gram.c"
     break;
 
   case 60:
 /* Line 1427 of yacc.c  */
-#line 482 "src/parse-gram.y"
+#line 483 "src/parse-gram.y"
     { (yyval.list) = symbol_list_prepend ((yyvsp[-1].list), symbol_list_sym_new ((yyvsp[0].symbol), (yylsp[0]))); }
 /* Line 1427 of yacc.c  */
-#line 2454 "src/parse-gram.c"
+#line 2461 "src/parse-gram.c"
     break;
 
   case 61:
 /* Line 1427 of yacc.c  */
-#line 486 "src/parse-gram.y"
+#line 487 "src/parse-gram.y"
     { (yyval.symbol) = (yyvsp[0].symbol); }
 /* Line 1427 of yacc.c  */
-#line 2462 "src/parse-gram.c"
+#line 2469 "src/parse-gram.c"
     break;
 
   case 62:
 /* Line 1427 of yacc.c  */
-#line 487 "src/parse-gram.y"
+#line 488 "src/parse-gram.y"
     { (yyval.symbol) = (yyvsp[-1].symbol); symbol_user_token_number_set ((yyvsp[-1].symbol), (yyvsp[0].integer), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2470 "src/parse-gram.c"
+#line 2477 "src/parse-gram.c"
     break;
 
   case 63:
 /* Line 1427 of yacc.c  */
-#line 493 "src/parse-gram.y"
+#line 494 "src/parse-gram.y"
     { (yyval.list) = symbol_list_sym_new ((yyvsp[0].symbol), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2478 "src/parse-gram.c"
+#line 2485 "src/parse-gram.c"
     break;
 
   case 64:
 /* Line 1427 of yacc.c  */
-#line 495 "src/parse-gram.y"
+#line 496 "src/parse-gram.y"
     { (yyval.list) = symbol_list_prepend ((yyvsp[-1].list), symbol_list_sym_new ((yyvsp[0].symbol), (yylsp[0]))); }
 /* Line 1427 of yacc.c  */
-#line 2486 "src/parse-gram.c"
+#line 2493 "src/parse-gram.c"
     break;
 
   case 65:
 /* Line 1427 of yacc.c  */
-#line 499 "src/parse-gram.y"
+#line 500 "src/parse-gram.y"
     { (yyval.list) = (yyvsp[0].list); }
 /* Line 1427 of yacc.c  */
-#line 2494 "src/parse-gram.c"
+#line 2501 "src/parse-gram.c"
     break;
 
   case 66:
 /* Line 1427 of yacc.c  */
-#line 500 "src/parse-gram.y"
+#line 501 "src/parse-gram.y"
     { (yyval.list) = symbol_list_prepend ((yyvsp[-1].list), (yyvsp[0].list)); }
 /* Line 1427 of yacc.c  */
-#line 2502 "src/parse-gram.c"
+#line 2509 "src/parse-gram.c"
     break;
 
   case 67:
 /* Line 1427 of yacc.c  */
-#line 504 "src/parse-gram.y"
+#line 505 "src/parse-gram.y"
     { (yyval.list) = symbol_list_sym_new ((yyvsp[0].symbol), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2510 "src/parse-gram.c"
+#line 2517 "src/parse-gram.c"
     break;
 
   case 68:
 /* Line 1427 of yacc.c  */
-#line 505 "src/parse-gram.y"
+#line 506 "src/parse-gram.y"
     { (yyval.list) = symbol_list_type_new ((yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2518 "src/parse-gram.c"
+#line 2525 "src/parse-gram.c"
     break;
 
   case 69:
 /* Line 1427 of yacc.c  */
-#line 506 "src/parse-gram.y"
+#line 507 "src/parse-gram.y"
     { (yyval.list) = symbol_list_default_tagged_new ((yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2526 "src/parse-gram.c"
+#line 2533 "src/parse-gram.c"
     break;
 
   case 70:
 /* Line 1427 of yacc.c  */
-#line 507 "src/parse-gram.y"
+#line 508 "src/parse-gram.y"
     { (yyval.list) = symbol_list_default_tagless_new ((yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2534 "src/parse-gram.c"
+#line 2541 "src/parse-gram.c"
     break;
 
   case 71:
 /* Line 1427 of yacc.c  */
-#line 513 "src/parse-gram.y"
+#line 514 "src/parse-gram.y"
     {
        current_type = (yyvsp[0].uniqstr);
        tag_seen = true;
      }
 /* Line 1427 of yacc.c  */
-#line 2545 "src/parse-gram.c"
+#line 2552 "src/parse-gram.c"
     break;
 
   case 72:
 /* Line 1427 of yacc.c  */
-#line 518 "src/parse-gram.y"
+#line 519 "src/parse-gram.y"
     {
        symbol_class_set ((yyvsp[0].symbol), current_class, (yylsp[0]), true);
        symbol_type_set ((yyvsp[0].symbol), current_type, (yylsp[0]));
      }
 /* Line 1427 of yacc.c  */
-#line 2556 "src/parse-gram.c"
+#line 2563 "src/parse-gram.c"
     break;
 
   case 73:
 /* Line 1427 of yacc.c  */
-#line 523 "src/parse-gram.y"
+#line 524 "src/parse-gram.y"
     {
       symbol_class_set ((yyvsp[-1].symbol), current_class, (yylsp[-1]), true);
       symbol_type_set ((yyvsp[-1].symbol), current_type, (yylsp[-1]));
       symbol_user_token_number_set ((yyvsp[-1].symbol), (yyvsp[0].integer), (yylsp[0]));
     }
 /* Line 1427 of yacc.c  */
-#line 2568 "src/parse-gram.c"
+#line 2575 "src/parse-gram.c"
     break;
 
   case 74:
 /* Line 1427 of yacc.c  */
-#line 529 "src/parse-gram.y"
+#line 530 "src/parse-gram.y"
     {
       symbol_class_set ((yyvsp[-1].symbol), current_class, (yylsp[-1]), true);
       symbol_type_set ((yyvsp[-1].symbol), current_type, (yylsp[-1]));
       symbol_make_alias ((yyvsp[-1].symbol), (yyvsp[0].symbol), (yyloc));
     }
 /* Line 1427 of yacc.c  */
-#line 2580 "src/parse-gram.c"
+#line 2587 "src/parse-gram.c"
     break;
 
   case 75:
 /* Line 1427 of yacc.c  */
-#line 535 "src/parse-gram.y"
+#line 536 "src/parse-gram.y"
     {
       symbol_class_set ((yyvsp[-2].symbol), current_class, (yylsp[-2]), true);
       symbol_type_set ((yyvsp[-2].symbol), current_type, (yylsp[-2]));
@@ -2589,136 +2596,144 @@ yyreduce:
       symbol_make_alias ((yyvsp[-2].symbol), (yyvsp[0].symbol), (yyloc));
     }
 /* Line 1427 of yacc.c  */
-#line 2593 "src/parse-gram.c"
+#line 2600 "src/parse-gram.c"
     break;
 
   case 82:
 /* Line 1427 of yacc.c  */
-#line 565 "src/parse-gram.y"
+#line 566 "src/parse-gram.y"
     {
       yyerrok;
     }
 /* Line 1427 of yacc.c  */
-#line 2603 "src/parse-gram.c"
+#line 2610 "src/parse-gram.c"
     break;
 
   case 83:
 /* Line 1427 of yacc.c  */
-#line 571 "src/parse-gram.y"
+#line 572 "src/parse-gram.y"
     { current_lhs = (yyvsp[-1].symbol); current_lhs_location = (yylsp[-1]);
     current_lhs_named_ref = (yyvsp[0].named_ref); }
 /* Line 1427 of yacc.c  */
-#line 2612 "src/parse-gram.c"
+#line 2619 "src/parse-gram.c"
     break;
 
   case 85:
 /* Line 1427 of yacc.c  */
-#line 576 "src/parse-gram.y"
+#line 577 "src/parse-gram.y"
     { grammar_current_rule_end ((yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2620 "src/parse-gram.c"
+#line 2627 "src/parse-gram.c"
     break;
 
   case 86:
 /* Line 1427 of yacc.c  */
-#line 577 "src/parse-gram.y"
+#line 578 "src/parse-gram.y"
     { grammar_current_rule_end ((yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2628 "src/parse-gram.c"
+#line 2635 "src/parse-gram.c"
     break;
 
   case 88:
 /* Line 1427 of yacc.c  */
-#line 583 "src/parse-gram.y"
+#line 584 "src/parse-gram.y"
     { grammar_current_rule_begin (current_lhs, current_lhs_location,
 				  current_lhs_named_ref); }
 /* Line 1427 of yacc.c  */
-#line 2637 "src/parse-gram.c"
+#line 2644 "src/parse-gram.c"
     break;
 
   case 89:
 /* Line 1427 of yacc.c  */
-#line 586 "src/parse-gram.y"
+#line 587 "src/parse-gram.y"
     { grammar_current_rule_symbol_append ((yyvsp[-1].symbol), (yylsp[-1]), (yyvsp[0].named_ref)); }
 /* Line 1427 of yacc.c  */
-#line 2645 "src/parse-gram.c"
+#line 2652 "src/parse-gram.c"
     break;
 
   case 90:
 /* Line 1427 of yacc.c  */
-#line 588 "src/parse-gram.y"
-    { grammar_current_rule_action_append ((yyvsp[-1].code), (yylsp[-1]), (yyvsp[0].named_ref)); }
+#line 589 "src/parse-gram.y"
+    { grammar_current_rule_action_append ((yyvsp[-1].code), (yylsp[-1]), (yyvsp[0].named_ref), false); }
 /* Line 1427 of yacc.c  */
-#line 2653 "src/parse-gram.c"
+#line 2660 "src/parse-gram.c"
     break;
 
   case 91:
 /* Line 1427 of yacc.c  */
-#line 590 "src/parse-gram.y"
-    { grammar_current_rule_prec_set ((yyvsp[0].symbol), (yylsp[0])); }
+#line 591 "src/parse-gram.y"
+    { grammar_current_rule_action_append ((yyvsp[0].code), (yylsp[0]), NULL, true); }
 /* Line 1427 of yacc.c  */
-#line 2661 "src/parse-gram.c"
+#line 2668 "src/parse-gram.c"
     break;
 
   case 92:
 /* Line 1427 of yacc.c  */
-#line 592 "src/parse-gram.y"
-    { grammar_current_rule_dprec_set ((yyvsp[0].integer), (yylsp[0])); }
+#line 593 "src/parse-gram.y"
+    { grammar_current_rule_prec_set ((yyvsp[0].symbol), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2669 "src/parse-gram.c"
+#line 2676 "src/parse-gram.c"
     break;
 
   case 93:
 /* Line 1427 of yacc.c  */
-#line 594 "src/parse-gram.y"
-    { grammar_current_rule_merge_set ((yyvsp[0].uniqstr), (yylsp[0])); }
+#line 595 "src/parse-gram.y"
+    { grammar_current_rule_dprec_set ((yyvsp[0].integer), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2677 "src/parse-gram.c"
+#line 2684 "src/parse-gram.c"
     break;
 
   case 94:
 /* Line 1427 of yacc.c  */
-#line 598 "src/parse-gram.y"
-    { (yyval.named_ref) = 0; }
+#line 597 "src/parse-gram.y"
+    { grammar_current_rule_merge_set ((yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2685 "src/parse-gram.c"
+#line 2692 "src/parse-gram.c"
     break;
 
   case 95:
 /* Line 1427 of yacc.c  */
-#line 600 "src/parse-gram.y"
+#line 601 "src/parse-gram.y"
+    { (yyval.named_ref) = 0; }
+/* Line 1427 of yacc.c  */
+#line 2700 "src/parse-gram.c"
+    break;
+
+  case 96:
+/* Line 1427 of yacc.c  */
+#line 603 "src/parse-gram.y"
     { (yyval.named_ref) = named_ref_new((yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2693 "src/parse-gram.c"
+#line 2708 "src/parse-gram.c"
     break;
 
-  case 97:
+  case 98:
 /* Line 1427 of yacc.c  */
-#line 612 "src/parse-gram.y"
+#line 614 "src/parse-gram.y"
     { (yyval.uniqstr) = uniqstr_new ((yyvsp[0].chars)); }
 /* Line 1427 of yacc.c  */
-#line 2701 "src/parse-gram.c"
+#line 2716 "src/parse-gram.c"
     break;
 
-  case 98:
+  case 99:
 /* Line 1427 of yacc.c  */
-#line 617 "src/parse-gram.y"
+#line 619 "src/parse-gram.y"
     { (yyval.chars) = ""; }
 /* Line 1427 of yacc.c  */
-#line 2709 "src/parse-gram.c"
+#line 2724 "src/parse-gram.c"
     break;
 
-  case 99:
+  case 100:
 /* Line 1427 of yacc.c  */
-#line 618 "src/parse-gram.y"
+#line 620 "src/parse-gram.y"
     { (yyval.chars) = (yyvsp[0].uniqstr); }
 /* Line 1427 of yacc.c  */
-#line 2717 "src/parse-gram.c"
+#line 2732 "src/parse-gram.c"
     break;
 
-  case 101:
+  case 102:
 /* Line 1427 of yacc.c  */
-#line 629 "src/parse-gram.y"
+#line 631 "src/parse-gram.y"
     {
       code_props plain_code;
       (yyvsp[0].code)[strlen ((yyvsp[0].code)) - 1] = '\n';
@@ -2728,51 +2743,51 @@ yyreduce:
       (yyval.chars) = plain_code.code;
     }
 /* Line 1427 of yacc.c  */
-#line 2732 "src/parse-gram.c"
+#line 2747 "src/parse-gram.c"
     break;
 
-  case 102:
+  case 103:
 /* Line 1427 of yacc.c  */
-#line 649 "src/parse-gram.y"
+#line 651 "src/parse-gram.y"
     { (yyval.symbol) = symbol_from_uniqstr ((yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2740 "src/parse-gram.c"
+#line 2755 "src/parse-gram.c"
     break;
 
-  case 103:
+  case 104:
 /* Line 1427 of yacc.c  */
-#line 651 "src/parse-gram.y"
+#line 653 "src/parse-gram.y"
     {
       (yyval.symbol) = symbol_get (char_name ((yyvsp[0].character)), (yylsp[0]));
       symbol_class_set ((yyval.symbol), token_sym, (yylsp[0]), false);
       symbol_user_token_number_set ((yyval.symbol), (yyvsp[0].character), (yylsp[0]));
     }
 /* Line 1427 of yacc.c  */
-#line 2752 "src/parse-gram.c"
+#line 2767 "src/parse-gram.c"
     break;
 
-  case 104:
+  case 105:
 /* Line 1427 of yacc.c  */
-#line 659 "src/parse-gram.y"
+#line 661 "src/parse-gram.y"
     { (yyval.symbol) = symbol_from_uniqstr ((yyvsp[0].uniqstr), (yylsp[0])); }
 /* Line 1427 of yacc.c  */
-#line 2760 "src/parse-gram.c"
+#line 2775 "src/parse-gram.c"
     break;
 
-  case 107:
+  case 108:
 /* Line 1427 of yacc.c  */
-#line 671 "src/parse-gram.y"
+#line 673 "src/parse-gram.y"
     {
       (yyval.symbol) = symbol_get (quotearg_style (c_quoting_style, (yyvsp[0].chars)), (yylsp[0]));
       symbol_class_set ((yyval.symbol), token_sym, (yylsp[0]), false);
     }
 /* Line 1427 of yacc.c  */
-#line 2771 "src/parse-gram.c"
+#line 2786 "src/parse-gram.c"
     break;
 
-  case 109:
+  case 110:
 /* Line 1427 of yacc.c  */
-#line 680 "src/parse-gram.y"
+#line 682 "src/parse-gram.y"
     {
       code_props plain_code;
       code_props_plain_init (&plain_code, (yyvsp[0].chars), (yylsp[0]));
@@ -2782,12 +2797,12 @@ yyreduce:
       code_scanner_last_string_free ();
     }
 /* Line 1427 of yacc.c  */
-#line 2786 "src/parse-gram.c"
+#line 2801 "src/parse-gram.c"
     break;
 
 
 /* Line 1427 of yacc.c  */
-#line 2791 "src/parse-gram.c"
+#line 2806 "src/parse-gram.c"
       default: break;
     }
   /* User semantic actions sometimes alter yychar, and that requires
@@ -3015,7 +3030,7 @@ yyreturn:
 }
 
 /* Line 1657 of yacc.c  */
-#line 690 "src/parse-gram.y"
+#line 692 "src/parse-gram.y"
 
 
 
diff --git a/src/parse-gram.h b/src/parse-gram.h
index 3363499a..d8eea03c 100644
--- a/src/parse-gram.h
+++ b/src/parse-gram.h
@@ -1,8 +1,9 @@
-/* A Bison parser, made by GNU Bison 2.4.483-4ad39-dirty.  */
+/* A Bison parser, made by GNU Bison 2.4.516-804e.  */
 
 /* Interface for Bison's Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2010 Free Software Foundation, Inc.
+   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
+   2007, 2008, 2009, 2010 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -31,7 +32,7 @@
    version 2.2 of Bison.  */
 /* "%code requires" blocks.  */
 /* Line 1658 of yacc.c  */
-#line 201 "src/parse-gram.y"
+#line 202 "src/parse-gram.y"
 
 # ifndef PARAM_TYPE
 #  define PARAM_TYPE
@@ -46,7 +47,7 @@
 
 
 /* Line 1658 of yacc.c  */
-#line 50 "src/parse-gram.h"
+#line 51 "src/parse-gram.h"
 
 /* Tokens.  */
 #ifndef YYTOKENTYPE
@@ -93,21 +94,22 @@
      PERCENT_VERBOSE = 293,
      PERCENT_YACC = 294,
      BRACED_CODE = 295,
-     BRACKETED_ID = 296,
-     CHAR = 297,
-     EPILOGUE = 298,
-     EQUAL = 299,
-     ID = 300,
-     ID_COLON = 301,
-     PERCENT_PERCENT = 302,
-     PIPE = 303,
-     PROLOGUE = 304,
-     SEMICOLON = 305,
-     TAG = 306,
-     TAG_ANY = 307,
-     TAG_NONE = 308,
-     PERCENT_PARAM = 309,
-     PERCENT_UNION = 310
+     BRACED_PREDICATE = 296,
+     BRACKETED_ID = 297,
+     CHAR = 298,
+     EPILOGUE = 299,
+     EQUAL = 300,
+     ID = 301,
+     ID_COLON = 302,
+     PERCENT_PERCENT = 303,
+     PIPE = 304,
+     PROLOGUE = 305,
+     SEMICOLON = 306,
+     TAG = 307,
+     TAG_ANY = 308,
+     TAG_NONE = 309,
+     PERCENT_PARAM = 310,
+     PERCENT_UNION = 311
    };
 #endif
 /* Tokens.  */
@@ -150,21 +152,22 @@
 #define PERCENT_VERBOSE 293
 #define PERCENT_YACC 294
 #define BRACED_CODE 295
-#define BRACKETED_ID 296
-#define CHAR 297
-#define EPILOGUE 298
-#define EQUAL 299
-#define ID 300
-#define ID_COLON 301
-#define PERCENT_PERCENT 302
-#define PIPE 303
-#define PROLOGUE 304
-#define SEMICOLON 305
-#define TAG 306
-#define TAG_ANY 307
-#define TAG_NONE 308
-#define PERCENT_PARAM 309
-#define PERCENT_UNION 310
+#define BRACED_PREDICATE 296
+#define BRACKETED_ID 297
+#define CHAR 298
+#define EPILOGUE 299
+#define EQUAL 300
+#define ID 301
+#define ID_COLON 302
+#define PERCENT_PERCENT 303
+#define PIPE 304
+#define PROLOGUE 305
+#define SEMICOLON 306
+#define TAG 307
+#define TAG_ANY 308
+#define TAG_NONE 309
+#define PERCENT_PARAM 310
+#define PERCENT_UNION 311
 
 
 
@@ -173,7 +176,7 @@
 typedef union YYSTYPE
 {
 /* Line 1658 of yacc.c  */
-#line 87 "src/parse-gram.y"
+#line 88 "src/parse-gram.y"
 
   assoc assoc;
   char *code;
@@ -186,13 +189,13 @@ typedef union YYSTYPE
   unsigned char character;
 
 /* Line 1658 of yacc.c  */
-#line 225 "src/parse-gram.y"
+#line 226 "src/parse-gram.y"
 
   param_type param;
 
 
 /* Line 1658 of yacc.c  */
-#line 196 "src/parse-gram.h"
+#line 199 "src/parse-gram.h"
 } YYSTYPE;
 # define YYSTYPE_IS_TRIVIAL 1
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
diff --git a/src/parse-gram.y b/src/parse-gram.y
index b897d691..d588dfa8 100644
--- a/src/parse-gram.y
+++ b/src/parse-gram.y
@@ -118,7 +118,6 @@ static char const *char_name (char);
 %token PERCENT_DPREC         "%dprec"
 %token PERCENT_MERGE         "%merge"
 
-
 /*----------------------.
 | Global Declarations.  |
 `----------------------*/
@@ -151,6 +150,7 @@ static char const *char_name (char);
 ;
 
 %token BRACED_CODE     "{...}"
+%token BRACED_PREDICATE "%?{...}"
 %token BRACKETED_ID    "[identifier]"
 %token CHAR            "char"
 %token EPILOGUE        "epilogue"
@@ -171,7 +171,7 @@ static char const *char_name (char);
 /* braceless is not to be used for rule or symbol actions, as it
    calls code_props_plain_init.  */
 %type <chars> STRING "%{...%}" EPILOGUE braceless content.opt
-%type <code> "{...}"
+%type <code> "{...}" "%?{...}"
 %printer { fputs (quotearg_style (c_quoting_style, $$), stderr); }
 	 STRING
 %printer { fprintf (stderr, "{\n%s\n}", $$); }
@@ -586,7 +586,9 @@ rhs:
 | rhs symbol named_ref.opt
     { grammar_current_rule_symbol_append ($2, @2, $3); }
 | rhs "{...}" named_ref.opt
-    { grammar_current_rule_action_append ($2, @2, $3); }
+    { grammar_current_rule_action_append ($2, @2, $3, false); }
+| rhs "%?{...}" 
+    { grammar_current_rule_action_append ($2, @2, NULL, true); }
 | rhs "%prec" symbol
     { grammar_current_rule_prec_set ($3, @3); }
 | rhs "%dprec" INT
@@ -601,7 +603,6 @@ named_ref.opt:
   BRACKETED_ID   { $$ = named_ref_new($1, @1); }
 ;
 
-
 /*---------------------------.
 | variable and content.opt.  |
 `---------------------------*/
diff --git a/src/reader.c b/src/reader.c
index 791fe181..e64741cb 100644
--- a/src/reader.c
+++ b/src/reader.c
@@ -377,7 +377,8 @@ grammar_midrule_action (void)
   code_props_rule_action_init (&midrule->action_props,
                                current_rule->action_props.code,
                                current_rule->action_props.location,
-                               midrule, 0);
+                               midrule, 0,
+			       current_rule->action_props.is_predicate);
   code_props_none_init (&current_rule->action_props);
 
   if (previous_rule_end)
@@ -468,14 +469,14 @@ grammar_current_rule_symbol_append (symbol *sym, location loc,
 
 void
 grammar_current_rule_action_append (const char *action, location loc,
-				    named_ref *name)
+				    named_ref *name, bool is_predicate)
 {
   if (current_rule->action_props.code)
     grammar_midrule_action ();
   /* After all symbol declarations have been parsed, packgram invokes
      code_props_translate_code.  */
   code_props_rule_action_init (&current_rule->action_props, action, loc,
-                               current_rule, name);
+                               current_rule, name, is_predicate);
 }
 
 
@@ -516,6 +517,7 @@ packgram (void)
       rules[ruleno].useful = true;
       rules[ruleno].action = p->action_props.code;
       rules[ruleno].action_location = p->action_props.location;
+      rules[ruleno].is_predicate = p->action_props.is_predicate;
 
       /* If the midrule's $$ is set or its $n is used, remove the `$' from the
 	 symbol name so that it's a user-defined symbol so that the default
diff --git a/src/reader.h b/src/reader.h
index 7d37f817..da8a6c7e 100644
--- a/src/reader.h
+++ b/src/reader.h
@@ -53,7 +53,7 @@ void grammar_current_rule_merge_set (uniqstr name, location loc);
 void grammar_current_rule_symbol_append (symbol *sym, location loc,
 					 named_ref *named_ref);
 void grammar_current_rule_action_append (const char *action, location loc,
-					 named_ref *named_ref);
+					 named_ref *named_ref, bool);
 void reader (void);
 void free_merger_functions (void);
 
diff --git a/src/scan-code.h b/src/scan-code.h
index 81a2afbe..79edc6e8 100644
--- a/src/scan-code.h
+++ b/src/scan-code.h
@@ -63,6 +63,13 @@ typedef struct code_props {
    */
   bool is_value_used;
 
+  /**
+   * \c true iff this code is an action that is not to be deferred in
+   * a non-deterministic parser.
+   */
+  bool is_predicate;
+
+
   /** \c NULL iff \c code_props::kind is not \c CODE_PROPS_RULE_ACTION.  */
   struct symbol_list *rule;
 
@@ -80,7 +87,7 @@ void code_props_none_init (code_props *self);
 
 /** Equivalent to \c code_props_none_init.  */
 #define CODE_PROPS_NONE_INIT \
-  {CODE_PROPS_NONE, NULL, EMPTY_LOCATION_INIT, false, NULL, NULL}
+  {CODE_PROPS_NONE, NULL, EMPTY_LOCATION_INIT, false, false, NULL, NULL}
 
 /** Initialized by \c CODE_PROPS_NONE_INIT with no further modification.  */
 extern code_props const code_props_none;
@@ -134,7 +141,7 @@ void code_props_symbol_action_init (code_props *self, char const *code,
  */
 void code_props_rule_action_init (code_props *self, char const *code,
                                   location code_loc, struct symbol_list *rule,
-                                  named_ref *name);
+                                  named_ref *name, bool is_predicate);
 
 /**
  * \pre
diff --git a/src/scan-code.l b/src/scan-code.l
index cb5d3a39..200fdf4e 100644
--- a/src/scan-code.l
+++ b/src/scan-code.l
@@ -902,7 +902,7 @@ code_props_symbol_action_init (code_props *self, char const *code,
 void
 code_props_rule_action_init (code_props *self, char const *code,
                              location code_loc, symbol_list *rule,
-			     named_ref *name)
+			     named_ref *name, bool is_predicate)
 {
   self->kind = CODE_PROPS_RULE_ACTION;
   self->code = code;
@@ -910,6 +910,7 @@ code_props_rule_action_init (code_props *self, char const *code,
   self->is_value_used = false;
   self->rule = rule;
   self->named_ref = name;
+  self->is_predicate = is_predicate;
 }
 
 void
diff --git a/src/scan-gram.l b/src/scan-gram.l
index 9d3d3428..462d007d 100644
--- a/src/scan-gram.l
+++ b/src/scan-gram.l
@@ -106,11 +106,12 @@ static void unexpected_newline (boundary, char const *);
  /* A complex tag, with nested angles brackets. */
 %x SC_TAG
 
- /* Three types of user code:
+ /* Four types of user code:
     - prologue (code between `%{' `%}' in the first section, before %%);
     - actions, printers, union, etc, (between braced in the middle section);
-    - epilogue (everything after the second %%). */
-%x SC_PROLOGUE SC_BRACED_CODE SC_EPILOGUE
+    - epilogue (everything after the second %%). 
+    - predicate (code between `%?{' and `{' in middle section); */
+%x SC_PROLOGUE SC_BRACED_CODE SC_EPILOGUE SC_PREDICATE
  /* C and C++ comments in code. */
 %x SC_COMMENT SC_LINE_COMMENT
  /* Strings and characters in code. */
@@ -285,6 +286,13 @@ splice	 (\\[ \f\t\v]*\n)*
     BEGIN SC_BRACED_CODE;
   }
 
+  /* Semantic predicate. */
+  "%?"[ \f\n\t\v]*"{" {
+    nesting = 0;
+    code_start = loc->start;
+    BEGIN SC_PREDICATE;
+  }
+
   /* A type. */
   "<*>"       return TAG_ANY;
   "<>"        return TAG_NONE;
@@ -661,7 +669,7 @@ splice	 (\\[ \f\t\v]*\n)*
   | Strings, comments etc. can be found in user code.  |
   `---------------------------------------------------*/
 
-<SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE>
+<SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE,SC_PREDICATE>
 {
   "'" {
     STRING_GROW;
@@ -691,14 +699,32 @@ splice	 (\\[ \f\t\v]*\n)*
 
 
   /*-----------------------------------------------------------.
-  | Scanning some code in braces (actions). The initial "{" is |
-  | already eaten.                                             |
+  | Scanning some code in braces (actions, predicates). The    |
+  | initial "{" is already eaten.                              |
   `-----------------------------------------------------------*/
 
-<SC_BRACED_CODE>
+<SC_BRACED_CODE,SC_PREDICATE>
 {
   "{"|"<"{splice}"%"  STRING_GROW; nesting++;
   "%"{splice}">"      STRING_GROW; nesting--;
+
+  /* Tokenize `<<%' correctly (as `<<' `%') rather than incorrrectly
+     (as `<' `<%').  */
+  "<"{splice}"<"  STRING_GROW;
+
+  <<EOF>> {
+    int token = (YY_START == SC_BRACED_CODE) ? BRACED_CODE : BRACED_PREDICATE;
+    unexpected_eof (code_start, "}");
+    STRING_FINISH;
+    loc->start = code_start;
+    val->code = last_string;
+    BEGIN INITIAL;
+    return token;
+  }
+}
+
+<SC_BRACED_CODE>
+{
   "}" {
     obstack_1grow (&obstack_for_string, '}');
 
@@ -712,22 +738,25 @@ splice	 (\\[ \f\t\v]*\n)*
 	return BRACED_CODE;
       }
   }
+}
 
-  /* Tokenize `<<%' correctly (as `<<' `%') rather than incorrrectly
-     (as `<' `<%').  */
-  "<"{splice}"<"  STRING_GROW;
-
-  <<EOF>> {
-    unexpected_eof (code_start, "}");
-    STRING_FINISH;
-    loc->start = code_start;
-    val->code = last_string;
-    BEGIN INITIAL;
-    return BRACED_CODE;
+<SC_PREDICATE>
+{
+  "}" {
+    --nesting;
+    if (nesting < 0)
+      {
+	STRING_FINISH;
+	loc->start = code_start;
+	val->code = last_string;
+	BEGIN INITIAL;
+	return BRACED_PREDICATE;
+      }
+    else
+      obstack_1grow (&obstack_for_string, '}');
   }
 }
 
-
   /*--------------------------------------------------------------.
   | Scanning some prologue: from "%{" (already scanned) to "%}".  |
   `--------------------------------------------------------------*/
@@ -774,8 +803,8 @@ splice	 (\\[ \f\t\v]*\n)*
   | By default, grow the string obstack with the input.  |
   `-----------------------------------------------------*/
 
-<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE,SC_STRING,SC_CHARACTER,SC_ESCAPED_STRING,SC_ESCAPED_CHARACTER>.	|
-<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE>\n	STRING_GROW;
+<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PREDICATE,SC_PROLOGUE,SC_EPILOGUE,SC_STRING,SC_CHARACTER,SC_ESCAPED_STRING,SC_ESCAPED_CHARACTER>.	|
+  <SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PREDICATE,SC_PROLOGUE,SC_EPILOGUE>\n	STRING_GROW;
 
 %%