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diff --git a/build-aux/Makefile.once.head/00-quote.mk b/build-aux/Makefile.once.head/00-quote.mk
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+# Copyright (C) 2016  Luke Shumaker
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, 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 Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+mod.quote.description = Macros to quote tricky strings
+define mod.quote.doc
+# Inputs:
+#   (none)
+# Outputs:
+#   - Global variable: `quote.var`       : GNU Make variables
+#   - Global variable: `quote.pattern`   : GNU Make patterns
+#   - Global variable: `quote.ere`       : POSIX Extended Regular Expressions
+#   - Global variable: `quote.bre`       : POSIX Basic Regular Expressions
+#   - Global variable: `quote.shell`     : POSIX sh(1) strings
+#   - Global variable: `quote.shell-each`: POSIX sh(1) strings
+#
+# Escaping/quoting things is hard!  This module provides a number of
+# functions to escape/quote strings for various contexts.
+#
+# `quote.shell-each` quotes each list-item separately (munging
+# whitespace), while `quote.shell` keeps them as one string (preserving
+# whitespace).
+endef
+mod.quote.doc := $(value mod.quote.doc)
+
+_quote.backslash = $(if $1,$(call _quote.backslash,$(wordlist 2,$(words $1),$1),$(subst $(firstword $1),\$(firstword $1),$2)),$2)
+
+quote.var     = $(subst $(at.nl),\$(at.nl),$(subst $$,$$$$,$1))
+quote.pattern = $(call _quote.backslash, \ % ,$1)
+quote.ere     = $(call _quote.backslash, \ ^ . [ $$ ( ) | * + ? { ,$1)
+quote.bre     = $(call _quote.backslash, \ ^ . [ $$       *       ,$1)
+
+quote.shell-each = $(foreach _quote.tmp,$1,$(call quote.shell,$(_quote.tmp)))
+
+# I put this as the last line in the file because it confuses Emacs
+# syntax highlighting and makes the remainder of the file difficult to
+# edit.
+quote.shell = $(subst $(at.nl),'$$'\n'','$(subst ','\'',$1)')