-Note that you must use &:include, which is an include processed during
-the generation of the per-directory Subdir.mk files. That ensures
-that the contents of these files is replicated, with appropriate
-per-directory substitutions, for each directory.
+Subdirectory templates `.sd.mk' vs plain autoconf templates `.mk.in'
+--------------------------------------------------------------------
+
+There are two kinds of template files.
+
+ Filename .sd.mk .mk.in
+
+ Processed by &-substitution, autoconf only
+ then autoconf
+
+ Need to be mentioned No, but Subdir.sd.mk All not in subdirmk/
+ in configure.ac via SUBDIRMK_SUBDIRS via SUBDIRMK_MAKEFILES
+
+ How to include `&:include foo.sd.mk' `include foo.mk'
+ in all relevant .sd.mk in only one
+ (but not needed for Subdir.sd.mk
+ Subdir and Perdir)
+
+If you `include subdirmk/regen.mk', dependency management and
+automatic regeneration for all of this template substitution, and for
+config.status etc. is done for you.