-# If the %define variable VARIABLE is defined to anything but "0" or "false",
-# expand IF-TRUE. If it is defined to "0" or "false", expand IF-FALSE. If
-# it is undefined, raise an error (this macro should be preceded by
-# b4_percent_define_default). Also, record the skeleton's usage of VARIABLE by
-# defining b4_percent_define_skeleton_variables(VARIABLE).
+# Mimic muscle_percent_define_flag_if in ../src/muscle_tab.h exactly. That is,
+# if the %define variable VARIABLE is defined to "" or "true", expand IF-TRUE.
+# If it is defined to "false", expand IF-FALSE. Complain if it is undefined
+# (a Bison or skeleton error since the default value should have been set
+# already) or defined to any other value (possibly a user error). Also, record
+# Bison's usage of VARIABLE by defining
+# b4_percent_define_bison_variables(VARIABLE).
+#
+# For example:
+#
+# b4_percent_define_flag_if([[foo]], [[it's true]], [[it's false]])