* src/scan-gram.l: When it starts with `%', complain about the

[bison.git] / data / m4sugar / m4sugar.m4

divert(-1)#                                                  -*- Autoconf -*-
# This file is part of Autoconf.
# Base M4 layer.
# Requires GNU M4.
# Copyright 1999, 2000, 2001, 2002 Free Software Foundation, Inc.
#
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# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2, or (at your option)
# any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
# 02111-1307, USA.
#
# As a special exception, the Free Software Foundation gives unlimited
# permission to copy, distribute and modify the configure scripts that
# are the output of Autoconf.  You need not follow the terms of the GNU
# General Public License when using or distributing such scripts, even
# though portions of the text of Autoconf appear in them.  The GNU
# General Public License (GPL) does govern all other use of the material
# that constitutes the Autoconf program.
#
# Certain portions of the Autoconf source text are designed to be copied
# (in certain cases, depending on the input) into the output of
# Autoconf.  We call these the "data" portions.  The rest of the Autoconf
# source text consists of comments plus executable code that decides which
# of the data portions to output in any given case.  We call these
# comments and executable code the "non-data" portions.  Autoconf never
# copies any of the non-data portions into its output.
#
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# the non-data portions to the data portions.)  If your modification has
# such potential, you must delete any notice of this special exception
# to the GPL from your modified version.
#
# Written by Akim Demaille.
#

# Set the quotes, whatever the current quoting system.
changequote()
changequote([, ])

# Some old m4's don't support m4exit.  But they provide
# equivalent functionality by core dumping because of the
# long macros we define.
ifdef([__gnu__], ,
[errprint(M4sugar requires GNU M4. Install it before installing M4sugar or
set the M4 environment variable to its path name.)
m4exit(2)])


## ------------------------------- ##
## 1. Simulate --prefix-builtins.  ##
## ------------------------------- ##

# m4_define
# m4_defn
# m4_undefine
define([m4_define],   defn([define]))
define([m4_defn],     defn([defn]))
define([m4_undefine], defn([undefine]))

m4_undefine([define])
m4_undefine([defn])
m4_undefine([undefine])


# m4_copy(SRC, DST)
# -----------------
# Define DST as the definition of SRC.
# What's the difference between:
# 1. m4_copy([from], [to])
# 2. m4_define([from], [to($@)])
# Well, obviously 1 is more expansive in space.  Maybe 2 is more expansive
# in time, but because of the space cost of 1, it's not that obvious.
# Nevertheless, one huge difference is the handling of `$0'.  If `from'
# uses `$0', then with 1, `to''s `$0' is `to', while it is `from' in 2.
# The user will certainly prefer see `from'.
m4_define([m4_copy],
[m4_define([$2], m4_defn([$1]))])


# m4_rename(SRC, DST)
# -------------------
# Rename the macro SRC as DST.
m4_define([m4_rename],
[m4_copy([$1], [$2])m4_undefine([$1])])


# m4_rename_m4(MACRO-NAME)
# ------------------------
# Rename MACRO-NAME as m4_MACRO-NAME.
m4_define([m4_rename_m4],
[m4_rename([$1], [m4_$1])])


# m4_copy_unm4(m4_MACRO-NAME)
# ---------------------------
# Copy m4_MACRO-NAME as MACRO-NAME.
m4_define([m4_copy_unm4],
[m4_copy([$1], m4_bpatsubst([$1], [^m4_\(.*\)], [[\1]]))])


# Some m4 internals have names colliding with tokens we might use.
# Rename them a` la `m4 --prefix-builtins'.
m4_rename_m4([builtin])
m4_rename_m4([changecom])
m4_rename_m4([changequote])
m4_rename_m4([debugfile])
m4_rename_m4([debugmode])
m4_rename_m4([decr])
m4_undefine([divert])
m4_rename_m4([divnum])
m4_rename_m4([dumpdef])
m4_rename_m4([errprint])
m4_rename_m4([esyscmd])
m4_rename_m4([eval])
m4_rename_m4([format])
m4_rename_m4([ifdef])
m4_rename([ifelse], [m4_if])
m4_rename_m4([include])
m4_rename_m4([incr])
m4_rename_m4([index])
m4_rename_m4([indir])
m4_rename_m4([len])
m4_rename([m4exit], [m4_exit])
m4_rename([m4wrap], [m4_wrap])
m4_rename_m4([maketemp])
m4_rename([patsubst], [m4_bpatsubst])
m4_undefine([popdef])
m4_rename_m4([pushdef])
m4_rename([regexp], [m4_bregexp])
m4_rename_m4([shift])
m4_rename_m4([sinclude])
m4_rename_m4([substr])
m4_rename_m4([symbols])
m4_rename_m4([syscmd])
m4_rename_m4([sysval])
m4_rename_m4([traceoff])
m4_rename_m4([traceon])
m4_rename_m4([translit])
m4_undefine([undivert])


## ------------------- ##
## 2. Error messages.  ##
## ------------------- ##


# m4_location
# -----------
m4_define([m4_location],
[__file__:__line__])


# m4_errprintn(MSG)
# -----------------
# Same as `errprint', but with the missing end of line.
m4_define([m4_errprintn],
[m4_errprint([$1
])])


# m4_warning(MSG)
# ---------------
# Warn the user.
m4_define([m4_warning],
[m4_errprintn(m4_location[: warning: $1])])


# m4_fatal(MSG, [EXIT-STATUS])
# ----------------------------
# Fatal the user.                                                      :)
m4_define([m4_fatal],
[m4_errprintn(m4_location[: error: $1])dnl
m4_expansion_stack_dump()dnl
m4_exit(m4_if([$2],, 1, [$2]))])


# m4_assert(EXPRESSION, [EXIT-STATUS = 1])
# ----------------------------------------
# This macro ensures that EXPRESSION evaluates to true, and exits if
# EXPRESSION evaluates to false.
m4_define([m4_assert],
[m4_if(m4_eval([$1]), 0,
       [m4_fatal([assert failed: $1], [$2])])])


## ------------- ##
## 3. Warnings.  ##
## ------------- ##


# m4_warning_ifelse(CATEGORY, IF-TRUE, IF-FALSE)
# ----------------------------------------------
# If the CATEGORY of warnings is enabled, expand IF_TRUE otherwise
# IF-FALSE.
#
# The variable `m4_warnings' contains a comma separated list of
# warnings which order is the converse from the one specified by
# the user, i.e., if she specified `-W error,none,obsolete',
# `m4_warnings' is `obsolete,none,error'.  We read it from left to
# right, and:
# - if none or noCATEGORY is met, run IF-FALSE
# - if all or CATEGORY is met, run IF-TRUE
# - if there is nothing left, run IF-FALSE.
m4_define([m4_warning_ifelse],
[_m4_warning_ifelse([$1], [$2], [$3], m4_warnings)])


# _m4_warning_ifelse(CATEGORY, IF-TRUE, IF-FALSE, WARNING1, ...)
# --------------------------------------------------------------
# Implementation of the loop described above.
m4_define([_m4_warning_ifelse],
[m4_case([$4],
         [$1],    [$2],
         [all],   [$2],
         [],      [$3],
         [none],  [$3],
         [no-$1], [$3],
         [$0([$1], [$2], [$3], m4_shiftn(4, $@))])])


# _m4_warning_error_ifelse(IF-TRUE, IF-FALSE)
# -------------------------------------------
# The same as m4_warning_ifelse, but scan for `error' only.
m4_define([_m4_warning_error_ifelse],
[__m4_warning_error_ifelse([$1], [$2], m4_warnings)])


# __m4_warning_error_ifelse(IF-TRUE, IF-FALSE)
# --------------------------------------------
# The same as _m4_warning_ifelse, but scan for `error' only.
m4_define([__m4_warning_error_ifelse],
[m4_case([$3],
         [error],    [$1],
         [],         [$2],
         [no-error], [$2],
         [$0([$1], [$2], m4_shiftn(3, $@))])])



# _m4_warn(MESSAGE)
# -----------------
# Report MESSAGE as a warning, unless the user requested -W error,
# in which case report a fatal error.
m4_define([_m4_warn],
[_m4_warning_error_ifelse([m4_fatal([$1])],
                          [m4_warning([$1])])])


# m4_warn(CATEGORY, MESSAGE)
# --------------------------
# Report a MESSAGE to the autoconf user if the CATEGORY of warnings
# is requested (in fact, not disabled).
m4_define([m4_warn],
[m4_warning_ifelse([$1], [_m4_warn([$2])])])




## ------------------- ##
## 4. File inclusion.  ##
## ------------------- ##


# We also want to neutralize include (and sinclude for symmetry),
# but we want to extend them slightly: warn when a file is included
# several times.  This is in general a dangerous operation because
# quite nobody quotes the first argument of m4_define.
#
# For instance in the following case:
#   m4_define(foo, [bar])
# then a second reading will turn into
#   m4_define(bar, [bar])
# which is certainly not what was meant.

# m4_include_unique(FILE)
# -----------------------
# Declare that the FILE was loading; and warn if it has already
# been included.
m4_define([m4_include_unique],
[m4_ifdef([m4_include($1)],
          [m4_warn([syntax], [file `$1' included several times])])dnl
m4_define([m4_include($1)])])


# m4_include(FILE)
# ----------------
# As the builtin include, but warns against multiple inclusions.
m4_define([m4_include],
[m4_include_unique([$1])dnl
m4_builtin([include], [$1])])


# m4_sinclude(FILE)
# -----------------
# As the builtin sinclude, but warns against multiple inclusions.
m4_define([m4_sinclude],
[m4_include_unique([$1])dnl
m4_builtin([sinclude], [$1])])



## ------------------------------------ ##
## 5. Additional branching constructs.  ##
## ------------------------------------ ##

# Both `m4_ifval' and `m4_ifset' tests against the empty string.  The
# difference is that `m4_ifset' is specialized on macros.
#
# In case of arguments of macros, eg $[1], it makes little difference.
# In the case of a macro `FOO', you don't want to check `m4_ifval(FOO,
# TRUE)', because if `FOO' expands with commas, there is a shifting of
# the arguments.  So you want to run `m4_ifval([FOO])', but then you just
# compare the *string* `FOO' against `', which, of course fails.
#
# So you want a variation of `m4_ifset' that expects a macro name as $[1].
# If this macro is both defined and defined to a non empty value, then
# it runs TRUE etc.


# m4_ifval(COND, [IF-TRUE], [IF-FALSE])
# -------------------------------------
# If COND is not the empty string, expand IF-TRUE, otherwise IF-FALSE.
# Comparable to m4_ifdef.
m4_define([m4_ifval],
[m4_if([$1], [], [$3], [$2])])


# m4_n(TEXT)
# ----------
# If TEXT is not empty, return TEXT and a new line, otherwise nothing.
m4_define([m4_n],
[m4_if([$1],
       [], [],
           [$1
])])


# m4_ifvaln(COND, [IF-TRUE], [IF-FALSE])
# --------------------------------------
# Same as `m4_ifval', but add an extra newline to IF-TRUE or IF-FALSE
# unless that argument is empty.
m4_define([m4_ifvaln],
[m4_if([$1],
       [],   [m4_n([$3])],
             [m4_n([$2])])])


# m4_ifset(MACRO, [IF-TRUE], [IF-FALSE])
# --------------------------------------
# If MACRO has no definition, or of its definition is the empty string,
# expand IF-FALSE, otherwise IF-TRUE.
m4_define([m4_ifset],
[m4_ifdef([$1],
          [m4_if(m4_defn([$1]), [], [$3], [$2])],
          [$3])])


# m4_ifndef(NAME, [IF-NOT-DEFINED], [IF-DEFINED])
# -----------------------------------------------
m4_define([m4_ifndef],
[m4_ifdef([$1], [$3], [$2])])


# m4_case(SWITCH, VAL1, IF-VAL1, VAL2, IF-VAL2, ..., DEFAULT)
# -----------------------------------------------------------
# m4 equivalent of
# switch (SWITCH)
# {
#   case VAL1:
#     IF-VAL1;
#     break;
#   case VAL2:
#     IF-VAL2;
#     break;
#   ...
#   default:
#     DEFAULT;
#     break;
# }.
# All the values are optional, and the macro is robust to active
# symbols properly quoted.
m4_define([m4_case],
[m4_if([$#], 0, [],
       [$#], 1, [],
       [$#], 2, [$2],
       [$1], [$2], [$3],
       [$0([$1], m4_shiftn(3, $@))])])


# m4_bmatch(SWITCH, RE1, VAL1, RE2, VAL2, ..., DEFAULT)
# -----------------------------------------------------
# m4 equivalent of
#
# if (SWITCH =~ RE1)
#   VAL1;
# elif (SWITCH =~ RE2)
#   VAL2;
# elif ...
#   ...
# else
#   DEFAULT
#
# All the values are optional, and the macro is robust to active symbols
# properly quoted.
m4_define([m4_bmatch],
[m4_if([$#], 0, [],
       [$#], 1, [],
       [$#], 2, [$2],
       [m4_if(m4_bregexp([$1], [$2]), -1, [$0([$1], m4_shiftn(3, $@))],
	      [$3])])])


# m4_map(MACRO, LIST)
# -------------------
# Invoke MACRO($1), MACRO($2) etc. where $1, $2... are the elements
# of LIST (which can be lists themselves, for multiple arguments MACROs).
m4_define([m4_fst], [$1])
m4_define([m4_map],
[m4_if([$2], [], [],
       [$2], [[]], [],
       [$1(m4_fst($2))[]dnl
m4_map([$1], m4_cdr($2))])])


# m4_map_sep(MACRO, SEPARATOR, LIST)
# ----------------------------------
# Invoke MACRO($1), SEPARATOR, MACRO($2), ..., MACRO($N) where $1, $2... $N
# are the elements of LIST (which can be lists themselves, for multiple
# arguments MACROs).
m4_define([m4_map_sep],
[m4_if([$3], [[]], [],
       [$1(m4_fst($3))[]dnl
m4_if(m4_cdr($3),
      [[]], [],
      [$2])[]dnl
m4_map_sep([$1], [$2], m4_cdr($3))])])


## ---------------------------------------- ##
## 6. Enhanced version of some primitives.  ##
## ---------------------------------------- ##

# m4_patsubsts(STRING, RE1, SUBST1, RE2, SUBST2, ...)
# ---------------------------------------------------
# m4 equivalent of
#
#   $_ = STRING;
#   s/RE1/SUBST1/g;
#   s/RE2/SUBST2/g;
#   ...
#
# All the values are optional, and the macro is robust to active symbols
# properly quoted.
#
# I would have liked to name this macro `m4_patsubst', unfortunately,
# due to quotation problems, I need to double quote $1 below, therefore
# the anchors are broken :(  I can't let users be trapped by that.
m4_define([m4_bpatsubsts],
[m4_if([$#], 0, [m4_fatal([$0: too few arguments: $#])],
       [$#], 1, [m4_fatal([$0: too few arguments: $#: $1])],
       [$#], 2, [m4_builtin([patsubst], $@)],
       [$0(m4_builtin([patsubst], [[$1]], [$2], [$3]),
           m4_shiftn(3, $@))])])



# m4_do(STRING, ...)
# ------------------
# This macro invokes all its arguments (in sequence, of course).  It is
# useful for making your macros more structured and readable by dropping
# unnecessary dnl's and have the macros indented properly.
m4_define([m4_do],
[m4_if($#, 0, [],
       $#, 1, [$1],
       [$1[]m4_do(m4_shift($@))])])


# m4_define_default(MACRO, VALUE)
# -------------------------------
# If MACRO is undefined, set it to VALUE.
m4_define([m4_define_default],
[m4_ifndef([$1], [m4_define($@)])])


# m4_default(EXP1, EXP2)
# ----------------------
# Returns EXP1 if non empty, otherwise EXP2.
m4_define([m4_default],
[m4_ifval([$1], [$1], [$2])])


# m4_defn(NAME)
# -------------
# Unlike to the original, don't tolerate popping something which is
# undefined.
m4_define([m4_defn],
[m4_ifndef([$1],
           [m4_fatal([$0: undefined macro: $1])])dnl
m4_builtin([defn], $@)])


# _m4_dumpdefs_up(NAME)
# ---------------------
m4_define([_m4_dumpdefs_up],
[m4_ifdef([$1],
          [m4_pushdef([_m4_dumpdefs], m4_defn([$1]))dnl
m4_dumpdef([$1])dnl
m4_popdef([$1])dnl
_m4_dumpdefs_up([$1])])])


# _m4_dumpdefs_down(NAME)
# -----------------------
m4_define([_m4_dumpdefs_down],
[m4_ifdef([_m4_dumpdefs],
          [m4_pushdef([$1], m4_defn([_m4_dumpdefs]))dnl
m4_popdef([_m4_dumpdefs])dnl
_m4_dumpdefs_down([$1])])])


# m4_dumpdefs(NAME)
# -----------------
# Similar to `m4_dumpdef(NAME)', but if NAME was m4_pushdef'ed, display its
# value stack (most recent displayed first).
m4_define([m4_dumpdefs],
[_m4_dumpdefs_up([$1])dnl
_m4_dumpdefs_down([$1])])


# m4_popdef(NAME)
# ---------------
# Unlike to the original, don't tolerate popping something which is
# undefined.
m4_define([m4_popdef],
[m4_ifndef([$1],
           [m4_fatal([$0: undefined macro: $1])])dnl
m4_builtin([popdef], $@)])


# m4_quote(ARGS)
# --------------
# Return ARGS as a single arguments.
#
# It is important to realize the difference between `m4_quote(exp)' and
# `[exp]': in the first case you obtain the quoted *result* of the
# expansion of EXP, while in the latter you just obtain the string
# `exp'.
m4_define([m4_quote],  [[$*]])
m4_define([m4_dquote],  [[$@]])


# m4_noquote(STRING)
# ------------------
# Return the result of ignoring all quotes in STRING and invoking the
# macros it contains.  Amongst other things useful for enabling macro
# invocations inside strings with [] blocks (for instance regexps and
# help-strings).
m4_define([m4_noquote],
[m4_changequote(-=<{,}>=-)$1-=<{}>=-m4_changequote([,])])


# m4_shiftn(N, ...)
# -----------------
# Returns ... shifted N times.  Useful for recursive "varargs" constructs.
m4_define([m4_shiftn],
[m4_assert(($1 >= 0) && ($# > $1))dnl
_m4_shiftn($@)])

m4_define([_m4_shiftn],
[m4_if([$1], 0,
       [m4_shift($@)],
       [_m4_shiftn(m4_eval([$1]-1), m4_shift(m4_shift($@)))])])


# m4_undefine(NAME)
# -----------------
# Unlike to the original, don't tolerate undefining something which is
# undefined.
m4_define([m4_undefine],
[m4_ifndef([$1],
           [m4_fatal([$0: undefined macro: $1])])dnl
m4_builtin([undefine], $@)])


## -------------------------- ##
## 7. Implementing m4 loops.  ##
## -------------------------- ##


# m4_for(VARIABLE, FIRST, LAST, [STEP = +/-1], EXPRESSION)
# --------------------------------------------------------
# Expand EXPRESSION defining VARIABLE to FROM, FROM + 1, ..., TO.
# Both limits are included, and bounds are checked for consistency.
m4_define([m4_for],
[m4_case(m4_sign(m4_eval($3 - $2)),
         1, [m4_assert(m4_sign(m4_default($4, 1)) == 1)],
        -1, [m4_assert(m4_sign(m4_default($4, -1)) == -1)])dnl
m4_pushdef([$1], [$2])dnl
m4_if(m4_eval([$3 > $2]), 1,
      [_m4_for([$1], [$3], m4_default([$4], 1), [$5])],
      [_m4_for([$1], [$3], m4_default([$4], -1), [$5])])dnl
m4_popdef([$1])])


# _m4_for(VARIABLE, FIRST, LAST, STEP, EXPRESSION)
# ------------------------------------------------
# Core of the loop, no consistency checks.
m4_define([_m4_for],
[$4[]dnl
m4_if($1, [$2], [],
      [m4_define([$1], m4_eval($1+[$3]))_m4_for([$1], [$2], [$3], [$4])])])


# Implementing `foreach' loops in m4 is much more tricky than it may
# seem.  Actually, the example of a `foreach' loop in the m4
# documentation is wrong: it does not quote the arguments properly,
# which leads to undesirable expansions.
#
# The example in the documentation is:
#
# | # foreach(VAR, (LIST), STMT)
# | m4_define([foreach],
# |        [m4_pushdef([$1])_foreach([$1], [$2], [$3])m4_popdef([$1])])
# | m4_define([_arg1], [$1])
# | m4_define([_foreach],
# | 	      [m4_if([$2], [()], ,
# | 		     [m4_define([$1], _arg1$2)$3[]_foreach([$1],
# |                                                        (shift$2),
# |                                                        [$3])])])
#
# But then if you run
#
# | m4_define(a, 1)
# | m4_define(b, 2)
# | m4_define(c, 3)
# | foreach([f], [([a], [(b], [c)])], [echo f
# | ])
#
# it gives
#
#  => echo 1
#  => echo (2,3)
#
# which is not what is expected.
#
# Of course the problem is that many quotes are missing.  So you add
# plenty of quotes at random places, until you reach the expected
# result.  Alternatively, if you are a quoting wizard, you directly
# reach the following implementation (but if you really did, then
# apply to the maintenance of m4sugar!).
#
# | # foreach(VAR, (LIST), STMT)
# | m4_define([foreach], [m4_pushdef([$1])_foreach($@)m4_popdef([$1])])
# | m4_define([_arg1], [[$1]])
# | m4_define([_foreach],
# |  [m4_if($2, [()], ,
# | 	    [m4_define([$1], [_arg1$2])$3[]_foreach([$1],
# |                                                 [(shift$2)],
# |                                                 [$3])])])
#
# which this time answers
#
#  => echo a
#  => echo (b
#  => echo c)
#
# Bingo!
#
# Well, not quite.
#
# With a better look, you realize that the parens are more a pain than
# a help: since anyway you need to quote properly the list, you end up
# with always using an outermost pair of parens and an outermost pair
# of quotes.  Rejecting the parens both eases the implementation, and
# simplifies the use:
#
# | # foreach(VAR, (LIST), STMT)
# | m4_define([foreach], [m4_pushdef([$1])_foreach($@)m4_popdef([$1])])
# | m4_define([_arg1], [$1])
# | m4_define([_foreach],
# |  [m4_if($2, [], ,
# | 	    [m4_define([$1], [_arg1($2)])$3[]_foreach([$1],
# |                                                   [shift($2)],
# |                                                   [$3])])])
#
#
# Now, just replace the `$2' with `m4_quote($2)' in the outer `m4_if'
# to improve robustness, and you come up with a quite satisfactory
# implementation.


# m4_foreach(VARIABLE, LIST, EXPRESSION)
# --------------------------------------
#
# Expand EXPRESSION assigning each value of the LIST to VARIABLE.
# LIST should have the form `item_1, item_2, ..., item_n', i.e. the
# whole list must *quoted*.  Quote members too if you don't want them
# to be expanded.
#
# This macro is robust to active symbols:
#      | m4_define(active, [ACT, IVE])
#      | m4_foreach(Var, [active, active], [-Var-])
#     => -ACT--IVE--ACT--IVE-
#
#      | m4_foreach(Var, [[active], [active]], [-Var-])
#     => -ACT, IVE--ACT, IVE-
#
#      | m4_foreach(Var, [[[active]], [[active]]], [-Var-])
#     => -active--active-
m4_define([m4_foreach],
[m4_pushdef([$1])_m4_foreach($@)m4_popdef([$1])])

# Low level macros used to define m4_foreach.
m4_define([m4_car], [[$1]])
m4_define([m4_cdr], [m4_dquote(m4_shift($@))])
m4_define([_m4_foreach],
[m4_if([$2], [[]], [],
       [m4_define([$1], m4_car($2))$3[]_m4_foreach([$1],
                                                   m4_cdr($2),
                                                   [$3])])])



## --------------------------- ##
## 8. More diversion support.  ##
## --------------------------- ##


# _m4_divert(DIVERSION-NAME or NUMBER)
# ------------------------------------
# If DIVERSION-NAME is the name of a diversion, return its number,
# otherwise if is a NUMBER return it.
m4_define([_m4_divert],
[m4_ifdef([_m4_divert($1)],
          [m4_indir([_m4_divert($1)])],
          [$1])])

# KILL is only used to suppress output.
m4_define([_m4_divert(KILL)],           -1)


# m4_divert(DIVERSION-NAME)
# -------------------------
# Change the diversion stream to DIVERSION-NAME.
m4_define([m4_divert],
[m4_define([m4_divert_stack],
           m4_location[: $0: $1]m4_ifdef([m4_divert_stack], [
m4_defn([m4_divert_stack])]))dnl
m4_builtin([divert], _m4_divert([$1]))dnl
])


# m4_divert_push(DIVERSION-NAME)
# ------------------------------
# Change the diversion stream to DIVERSION-NAME, while stacking old values.
m4_define([m4_divert_push],
[m4_pushdef([m4_divert_stack],
            m4_location[: $0: $1]m4_ifdef([m4_divert_stack], [
m4_defn([m4_divert_stack])]))dnl
m4_pushdef([_m4_divert_diversion], [$1])dnl
m4_builtin([divert], _m4_divert(_m4_divert_diversion))dnl
])


# m4_divert_pop([DIVERSION-NAME])
# -------------------------------
# Change the diversion stream to its previous value, unstacking it.
# If specified, verify we left DIVERSION-NAME.
m4_define([m4_divert_pop],
[m4_ifval([$1],
     [m4_if(_m4_divert([$1]), m4_divnum, [],
            [m4_fatal([$0($1): diversion mismatch: ]
m4_defn([m4_divert_stack]))])])dnl
m4_popdef([_m4_divert_diversion])dnl
dnl m4_ifndef([_m4_divert_diversion],
dnl           [m4_fatal([too many m4_divert_pop])])dnl
m4_builtin([divert],
           m4_ifdef([_m4_divert_diversion],
                    [_m4_divert(_m4_divert_diversion)], -1))dnl
m4_popdef([m4_divert_stack])dnl
])


# m4_divert_text(DIVERSION-NAME, CONTENT)
# ---------------------------------------
# Output CONTENT into DIVERSION-NAME (which may be a number actually).
# An end of line is appended for free to CONTENT.
m4_define([m4_divert_text],
[m4_divert_push([$1])dnl
$2
m4_divert_pop([$1])dnl
])


# m4_divert_once(DIVERSION-NAME, CONTENT)
# ---------------------------------------
# Output once CONTENT into DIVERSION-NAME (which may be a number
# actually).  An end of line is appended for free to CONTENT.
m4_define([m4_divert_once],
[m4_expand_once([m4_divert_text([$1], [$2])])])


# m4_undivert(DIVERSION-NAME)
# ---------------------------
# Undivert DIVERSION-NAME.
m4_define([m4_undivert],
[m4_builtin([undivert], _m4_divert([$1]))])


## -------------------------------------------- ##
## 8. Defining macros with bells and whistles.  ##
## -------------------------------------------- ##

# `m4_defun' is basically `m4_define' but it equips the macro with the
# needed machinery for `m4_require'.  A macro must be m4_defun'd if
# either it is m4_require'd, or it m4_require's.
#
# Two things deserve attention and are detailed below:
#  1. Implementation of m4_require
#  2. Keeping track of the expansion stack
#
# 1. Implementation of m4_require
# ===============================
#
# Of course m4_defun AC_PROVIDE's the macro, so that a macro which has
# been expanded is not expanded again when m4_require'd, but the
# difficult part is the proper expansion of macros when they are
# m4_require'd.
#
# The implementation is based on two ideas, (i) using diversions to