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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.var.description = Depend on the values of variables
+define mod.var.doc
+# Inputs:
+# (user-defined)
+# Outputs:
+# Target : `$(outdir)/.var.%`
+# Directory variable: `at.targets`
+#
+# It's a well-known secret that many files generated by a Makefile vary with
+# the values of particular variables, but that GNU Make can't track these
+# dependencies. Well, with some cleverness, it actually can!
+#
+# With this module, to depend on the value of a variable, depend on
+# `$(outdir)/.var.VARNAME`.
+endef
+mod.var.doc := $(value mod.var.doc)
+
+.PHONY: _var.FORCE