+
+# b4_percent_define_if_define(VARIABLE)
+# -------------------------------------
+# Define b4_VARIABLE_if that executes its $1 or $2 depending whether VARIABLE
+# was %defined.
+m4_define([b4_percent_define_if_define_],
+[m4_define([b4_$1_if], [b4_percent_define_ifdef([$1], [$2], [$3])])])
+m4_define([b4_percent_define_if_define],
+[b4_percent_define_if_define_([$1], $[1], $[2])])
+
+## --------- ##
+## Options. ##
+## --------- ##
+
+
+# b4_assert_if([IF-ASSERTIONS-ARE-USED], [IF-NOT])
+# b4_lex_symbol_if([IF-YYLEX-RETURNS-A-COMPLETE-SYMBOL], [IF-NOT])
+# b4_variant_if([IF-VARIANT-ARE-USED], [IF-NOT])
+# ----------------------------------------------
+b4_percent_define_if_define([assert])
+b4_percent_define_if_define([lex_symbol])
+b4_percent_define_if_define([variant])
+
+
+