(systemd) Autothing documentation

15 files changed, 523 insertions, 168 deletions

diff --git a/build-aux/Makefile.README.old.txt b/build-aux/Makefile.README.old.txt
new file mode 100644
index 0000000..b0a0965
--- /dev/null
+++ b/build-aux/Makefile.README.old.txt
@@ -0,0 +1,161 @@
+Obsolete
+========
+
+The following was written for previous versions of Autothing.  I'm leaving it
+here for now because I'll likely canibalize it for other bits of documentation,
+either for Autothing itself, the `files` module, or the `dist` module.
+
+High-level overview
+-------------------
+
+Now, what this does for you is:
+
+ (search for the paper
+"Recursive Make Considered Harmful")  As harmful as recursive make is,
+it's historically been difficult to to write non-recursive Makefiles.
+This makes it easy.
+
+It also makes it easy to follow the GNU standards for your makefiles:
+it takes care of this entire table of .PHONY targets for you:
+
+| this | and this         | are aliases for this                                   |
+|------+------------------+--------------------------------------------------------|
+| all  | build            | $(outdir)/build                                        |
+|      | install          | $(outdir)/install                                      |
+|      | uninstall        | $(outdir)/uninstall                                    |
+|      | mostlyclean      | $(outdir)/mostlyclean                                  |
+|      | clean            | $(outdir)/clean                                        |
+|      | distclean        | $(outdir)/distclean                                    |
+|      | maintainer-clean | $(outdir)/maintainer-clean                             |
+|      | check            | $(outdir)/check (not implemented for you)              |
+|      | dist             | $(topoutdir)/$(PACKAGE)-$(VERSION).tar.gz (not .PHONY) |
+
+(You are still responsible for implementing the `$(outdir)/check`
+target in each of your Makefiles.)
+
+What you have to do is:
+
+In each source directory, you write a `Makefile`, very similarly to if
+you were writing for plain GNU Make, with
+
+    topoutdir ?= ...
+    topsrcdir ?= ...
+    include $(topsrcdir)/build-aux/Makefile.head.mk
+
+    # your makefile
+
+    include $(topsrcdir)/build-aux/Makefile.tail.mk
+
+And in the top-level source directory, Write your own helper makefiles
+that get included:
+ - `common.once.head.mk`: before parsing any of your Makefiles
+ - `common.each.head.mk`: before parsing each of your Makefiles
+ - `common.each.tail.mk`: after parsing each of your Makefiles
+ - `common.each.tail.mk`: after parsing all of your Makefiles
+
+The `common.*.mk` makefiles are nice for including generic pattern
+rules and variables that aren't specific to a directory.
+
+You're probably thinking that this sounds too good to be true!
+Unfortunately, there are two major deviations from writing a plain
+recursive Makefile:
+
+ 1. all targets and prerequisites (including .PHONY targets!) need to
+    be prefixed with
+    `$(srcdir)`/`$(outdir)`/`$(topsrcdir)`/`$(topoutdir)`.
+      * sub-gotcha: this means that if a pattern rule has a
+        prerequisite that may be in srcdir or outdir, then it must be
+        specified twice, once for each case.
+ 2. if a prerequisite is in a directory "owned" by another Makefile,
+    you must filter the pathname through `am_path`:
+    `$(call am_path,YOUR_PATH)`.  Further, that path must NOT contain
+    a `..` segment; if you need to refer to a sibling directory, do it
+    relative to `$(topoutdir)` or `$(topsrcdir)`.
+
+Telling automake about your program
+-----------------------------------
+
+You tell automake what to do for you by setting some variables.  They
+are all prefixed with `am_`; this prefix may be changed by editing the
+`_am` variable at the top of `automake.head.mk`.
+
+The exception to this is the `am_path` variable, which is a macro that
+is used to make a list of filenames relative to the appropriate
+directory, because unlike normal GNU (Auto)Make, `$(outdir)` isn't
+nescessarily equal to `.`.  See above.
+
+There are several commands that generate files; simply record the list
+of files that each command generates as the following variable
+variables:
+
+| Variable     | Create Command | Delete Command              | Description                       | Relative to |
+|--------------+----------------+-----------------------------+-----------------------------------+-------------|
+| am_src_files | emacs          | rm -rf .                    | Files that the developer writes   | srcdir      |
+| am_gen_files | ???            | make maintainer-clean       | Files the developer compiles      | srcdir      |
+| am_cfg_files | ./configure    | make distclean              | Users' compile-time configuration | outdir      |
+| am_out_files | make all       | make mostlyclean/make clean | Files the user compiles           | outdir      |
+| am_sys_files | make install   | make uninstall              | Files the user installs           | DESTDIR     |
+
+In addition, there are two more variables that control not how files
+are created, but how they are deleted:
+
+| Variable       | Affected command | Description                                    | Relative to |
+|----------------+------------------+------------------------------------------------+-------------|
+| am_clean_files | make clean       | A list of things to `rm` in addition to the    | outdir      |
+|                |                  | files in `$(am_out_files)`.  (Example: `*.o`)  |             |
+|----------------+------------------+------------------------------------------------+-------------|
+| am_slow_files  | make mostlyclean | A list of things that (as an exception) should | outdir      |
+|                |                  | _not_ be deleted.  (otherwise, `mostlyclean`   |             |
+|                |                  | is the same as `clean`)                        |             |
+
+Finally, there are two variables that express the relationships
+between directories:
+
+| Variable   | Description                                             |
+|------------+---------------------------------------------------------|
+| am_subdirs | A list of other directories (containing Makefiles) that |
+|            | may be considered "children" of this                    |
+|            | directory/Makefile; building a phony target in this     |
+|            | directory should also build it in the subdirectory.     |
+|            | They are not necesarily actually subdirectories of this |
+|            | directory in the filesystem.                            |
+|------------+---------------------------------------------------------|
+| am_depdirs | A list of other directories (containing Makefiles) that |
+|            | contain or generate files that are dependencies of      |
+|            | targets in this directory.  They are not necesarily     |
+|            | actually subdirectories of this directory in the        |
+|            | filesystem.  Except for files that are dependencies of  |
+|            | files in this directory, things in the dependency       |
+|            | directory will not be built.                            |
+
+Tips, notes
+-----------
+
+I like to have the first (non-comment) line in a Makefile be:
+
+    include $(dir $(lastword $(MAKEFILE_LIST)))/../../config.mk
+
+(adjusting the number of `../` sequences as nescessary).  Then, my
+(user-editable) `config.mk` is of the form:
+
+    ifeq ($(topsrcdir),)
+    topoutdir := $(patsubst %/,%,$(dir $(lastword $(MAKEFILE_LIST))))
+    topsrcdir := $(topoutdir)
+
+    # your configuration
+
+    endif
+
+If the package has a `./configure` script, then I have it modifiy
+topsrcdir as necessary, as well as modifying whatever other parts of
+the configuration.  All of the configuration lives in `config.mk`;
+`./configure` doesn't modify any `Makefile`s, it just generates
+`config.mk`, and copies (or (sym?)link?) every `$(srcdir)/Makefile` to
+`$(outdir)/Makefile`.
+
+----
+Copyright (C) 2016  Luke Shumaker
+
+This documentation file is placed into the public domain.  If that is
+not possible in your legal system, I grant you permission to use it in
+absolutely every way that I can legally grant to you.
diff --git a/build-aux/Makefile.README.txt b/build-aux/Makefile.README.txt
index e06ba52..3be7a9d 100644
--- a/build-aux/Makefile.README.txt
+++ b/build-aux/Makefile.README.txt
@@ -1,160 +1,108 @@
-Luke's AutoMake
-===============
-
-Yo, this document is incomplete.  It describes the magical
-automake.{head,tail}.mk Makefiles and how to use them, kinda.
-
-I wrote a "clone" of automake.  I say clone, because it works
-differently.  Yeah, I need a new name for it.
-
-High-level overview
--------------------
-
-Now, what this does for you is:
-
-It makes it _easy_ to write non-recursive Makefiles--and ones that are
-similar to plain recursive Makefiles, at that! (search for the paper
-"Recursive Make Considered Harmful")  As harmful as recursive make is,
-it's historically been difficult to to write non-recursive Makefiles.
-This makes it easy.
-
-It also makes it easy to follow the GNU standards for your makefiles:
-it takes care of this entire table of .PHONY targets for you:
-
-| this | and this         | are aliases for this                                   |
-|------+------------------+--------------------------------------------------------|
-| all  | build            | $(outdir)/build                                        |
-|      | install          | $(outdir)/install                                      |
-|      | uninstall        | $(outdir)/uninstall                                    |
-|      | mostlyclean      | $(outdir)/mostlyclean                                  |
-|      | clean            | $(outdir)/clean                                        |
-|      | distclean        | $(outdir)/distclean                                    |
-|      | maintainer-clean | $(outdir)/maintainer-clean                             |
-|      | check            | $(outdir)/check (not implemented for you)              |
-|      | dist             | $(topoutdir)/$(PACKAGE)-$(VERSION).tar.gz (not .PHONY) |
-
-(You are still responsible for implementing the `$(outdir)/check`
-target in each of your Makefiles.)
-
-What you have to do is:
-
-In each source directory, you write a `Makefile`, very similarly to if
-you were writing for plain GNU Make, with
-
-    topoutdir ?= ...
-    topsrcdir ?= ...
-    include $(topsrcdir)/build-aux/Makefile.head.mk
-
-    # your makefile
-    
-    include $(topsrcdir)/build-aux/Makefile.tail.mk
-
-And in the top-level source directory, Write your own helper makefiles
-that get included:
- - `common.once.head.mk`: before parsing any of your Makefiles
- - `common.each.head.mk`: before parsing each of your Makefiles
- - `common.each.tail.mk`: after parsing each of your Makefiles
- - `common.each.tail.mk`: after parsing all of your Makefiles
-
-The `common.*.mk` makefiles are nice for including generic pattern
-rules and variables that aren't specific to a directory.
-
-You're probably thinking that this sounds too good to be true!
-Unfortunately, there are two major deviations from writing a plain
-recursive Makefile:
-
- 1. all targets and prerequisites (including .PHONY targets!) need to
-    be prefixed with
-    `$(srcdir)`/`$(outdir)`/`$(topsrcdir)`/`$(topoutdir)`.
-      * sub-gotcha: this means that if a pattern rule has a
-        prerequisite that may be in srcdir or outdir, then it must be
-        specified twice, once for each case.
- 2. if a prerequisite is in a directory "owned" by another Makefile,
-    you must filter the pathname through `am_path`:
-    `$(call am_path,YOUR_PATH)`.  Further, that path must NOT contain
-    a `..` segment; if you need to refer to a sibling directory, do it
-    relative to `$(topoutdir)` or `$(topsrcdir)`.
-
-Telling automake about your program
------------------------------------
-
-You tell automake what to do for you by setting some variables.  They
-are all prefixed with `am_`; this prefix may be changed by editing the
-`_am` variable at the top of `automake.head.mk`.
-
-The exception to this is the `am_path` variable, which is a macro that
-is used to make a list of filenames relative to the appropriate
-directory, because unlike normal GNU (Auto)Make, `$(outdir)` isn't
-nescessarily equal to `.`.  See above.
-
-There are several commands that generate files; simply record the list
-of files that each command generates as the following variable
-variables:
-
-| Variable     | Create Command | Delete Command              | Description                       | Relative to |
-|--------------+----------------+-----------------------------+-----------------------------------+-------------|
-| am_src_files | emacs          | rm -rf .                    | Files that the developer writes   | srcdir      |
-| am_gen_files | ???            | make maintainer-clean       | Files the developer compiles      | srcdir      |
-| am_cfg_files | ./configure    | make distclean              | Users' compile-time configuration | outdir      |
-| am_out_files | make all       | make mostlyclean/make clean | Files the user compiles           | outdir      |
-| am_sys_files | make install   | make uninstall              | Files the user installs           | DESTDIR     |
-
-In addition, there are two more variables that control not how files
-are created, but how they are deleted:
-
-| Variable       | Affected command | Description                                    | Relative to |
-|----------------+------------------+------------------------------------------------+-------------|
-| am_clean_files | make clean       | A list of things to `rm` in addition to the    | outdir      |
-|                |                  | files in `$(am_out_files)`.  (Example: `*.o`)  |             |
-|----------------+------------------+------------------------------------------------+-------------|
-| am_slow_files  | make mostlyclean | A list of things that (as an exception) should | outdir      |
-|                |                  | _not_ be deleted.  (otherwise, `mostlyclean`   |             |
-|                |                  | is the same as `clean`)                        |             |
-
-Finally, there are two variables that express the relationships
-between directories:
-
-| Variable   | Description                                             |
-|------------+---------------------------------------------------------|
-| am_subdirs | A list of other directories (containing Makefiles) that |
-|            | may be considered "children" of this                    |
-|            | directory/Makefile; building a phony target in this     |
-|            | directory should also build it in the subdirectory.     |
-|            | They are not necesarily actually subdirectories of this |
-|            | directory in the filesystem.                            |
-|------------+---------------------------------------------------------|
-| am_depdirs | A list of other directories (containing Makefiles) that |
-|            | contain or generate files that are dependencies of      |
-|            | targets in this directory.  They are not necesarily     |
-|            | actually subdirectories of this directory in the        |
-|            | filesystem.  Except for files that are dependencies of  |
-|            | files in this directory, things in the dependency       |
-|            | directory will not be built.                            |
-
-Tips, notes
------------
-
-I like to have the first (non-comment) line in a Makefile be:
-
-    include $(dir $(lastword $(MAKEFILE_LIST)))/../../config.mk
-
-(adjusting the number of `../` sequences as nescessary).  Then, my
-(user-editable) `config.mk` is of the form:
-
-    ifeq ($(topsrcdir),)
-    topoutdir := $(patsubst %/,%,$(dir $(lastword $(MAKEFILE_LIST))))
-    topsrcdir := $(topoutdir)
-
-    # your configuration
-
-    endif
-
-If the package has a `./configure` script, then I have it modifiy
-topsrcdir as necessary, as well as modifying whatever other parts of
-the configuration.  All of the configuration lives in `config.mk`;
-`./configure` doesn't modify any `Makefile`s, it just generates
-`config.mk`, and copies (or (sym?)link?) every `$(srcdir)/Makefile` to
-`$(outdir)/Makefile`.
+# -*- Mode: markdown -*-
+
+Autothing 3: The smart way to write GNU Makefiles
+=================================================
+
+Autothing is a thing that does things automatically.
+
+Ok, more helpfully: Autothing is a pair of .mk Makefile fragments that
+you can `include` from your Makefiles to make them easier to write;
+specifically, it makes it _easy_ to write non-recursive Makefiles--and
+ones that are similar to plain recursive Makefiles, at that!
+
+Synopsis
+--------
+
+Write your makefiles of the form:
+
+	topsrcdir ?= ...
+	topoutdir ?= ...
+	at.Makefile ?= Makefile # Optional
+	include $(topsrcdir)/build-aux/Makefile.head.mk
+
+	$(outdir)/%.o: $(srcdir)/%.c:
+                $(CC) -c -o $@ $<
+
+        $(outdir)/hello: $(outdir)/hello.o
+
+	at.subdirs = ...
+	at.targets = ...
+
+	include $(topsrcdir)/build-aux/Makefile.tail.mk
+
+This is similar to, but not quite, the comfortable way that you probably
+already write your Makefiles.
+
+Details
+-------
+
+There are two fundamental things that Autothing provides:
+
+ 1. Variable namespacing
+ 2. Tools for dealing with paths
+
+The first is important because globals are bad for composability.
+
+The second is important because GNU Make is too dumb to know that
+`foo/bar/../baz` == `foo/baz`.
+
+Then, there's something that maybe doesn't belong, but I didn't have the heart
+to cut it out:
+
+ 3. A module (plugin) system.
+
+The module system is "important" because there are very often common bits that
+you want to be included in every Makefile, and this gives some structure to
+that.
+
+Requirements:
+  - A version of GNU Make that supports `undefine`. (TODO: check which version
+    of GNU Make introduced this feature)
+
+Inputs:
+  - In each `Makefile`:
+    - Before `Makefile.head.mk`:
+      - Variable (mandatory) : `topoutdir`
+      - Variable (mandatory) : `topsrcdir` (must not be a subdirectory of `$(topoutdir)`)
+      - Variable (optional)  : `at.Makefile` (Default: `Makefile`)
+    - Between `Makefile.head.mk` and `Makefile.tail.mk`:
+      - Variable: `at.targets` (Default: empty)
+      - Variable: `at.subdirs` (Default: empty)
+  - Files:
+      - `${topsrcdir}/build-aux/Makefile.{each,once}.{head,tail}/*.mk`
+
+Outputs:
+  - Global:
+    - Variable (function): `$(call at.is_subdir,        parent, child)`
+    - Variable (function): `$(call at.is_strict_subdir, parent, child)`
+    - Variable (function): `$(call at.relbase,          parent, children...)`
+    - Variable (function): `$(call at.relto,            parent, children...)`
+    - Variable (function): `$(call at.path, paths...)`
+    - Variable (function): `$(call at.out2src, paths...)`
+    - Variable (function): `$(call at.addprefix, prefix, paths...)`
+  - Per-directory:
+    - Variable: `$(outdir)`
+    - Variable: `$(srcdir)`
+
+TODO: actually explain things.
+
+Motivation/Exposition
+---------------------
+
+This section needs rewritten.  Or really just written.
+
+Other projects like GNU Automake were created to plaster over differences
+between make(1) implementations; however, this isn't all that Automake
+provides, it also makes it easy to do complex things that users want, or the
+GNU Coding Standards require.  That's silly.
+
+Autothing does depend on GNU Make; other make(1) implementations will
+not work.  However, if you are open to adding GNU Make as a
+dependency, then Autothing should obviate the need for GNU Automake,
+while also making your Makefiles better.
+
+  Peter Miller (1997) "Recursive Make Considered Harmful"
+    <http://aegis.sourceforge.net/auug97.pdf>
 
 ----
 Copyright (C) 2016  Luke Shumaker
diff --git a/build-aux/Makefile.each.tail/00-dist.mk b/build-aux/Makefile.each.tail/00-dist.mk
index df363b5..bc2a3d5 100644
--- a/build-aux/Makefile.each.tail/00-dist.mk
+++ b/build-aux/Makefile.each.tail/00-dist.mk
@@ -1 +1 @@
-_dist.files := $(strip $(_dist.files) $(_files.src))
+_dist.files := $(strip $(_dist.files) $(call at.addprefix,$(srcdir),$(files.src)))
diff --git a/build-aux/Makefile.each.tail/00-mod.mk b/build-aux/Makefile.each.tail/00-mod.mk
index dc1a2fe..5b77436 100644
--- a/build-aux/Makefile.each.tail/00-mod.mk
+++ b/build-aux/Makefile.each.tail/00-mod.mk
@@ -29,12 +29,15 @@ at.targets += $(addprefix $(outdir)/, at-variables-global at-variables-local at-
 
 $(outdir)/at-modules:
 	@printf 'Autothing modules used in this project:\n'
-	@printf ' - %s\n' $(foreach _mod.tmp,$(_mod.modules),$(call quote.shell,$(_mod.tmp)	$(mod.$(_mod.tmp).description)))|column -t -s $$'\t'
+	@printf ' - %s\n' $(foreach _mod.tmp,$(_mod.modules),$(call quote.shell,$(_mod.tmp)	$(mod.$(_mod.tmp).description)	$(if $(value mod.$(_mod.tmp).doc),(more))))|column -t -s $$'\t'
 $(addprefix $(outdir)/at-modules/,$(_mod.modules)): $(outdir)/at-modules/%:
-	@printf 'Name           : %s\n' $(call quote.shell,$*)
-	@printf 'Description    : %s\n' $(call quote.shell,$(mod.$*.description))
-	@echo   'Contains Files :' $(call quote.shell-each,$(call at.relto,$(topsrcdir),$(sort $(mod.$*.files) $(wildcard $(topsrcdir)/build-aux/Makefile.*/??-$*.mk))))
-	@echo   'Depends on     :' $(mod.$*.depends)
+	@printf 'Name          : %s\n' $(call quote.shell,$*)
+	@printf 'Description   : %s\n' $(call quote.shell,$(mod.$*.description))
+	@echo   'Depends on    :' $(sort $(mod.$*.depends))
+	@echo   'Files         :'
+	@printf '  %s\n' $(call quote.shell-each,$(call at.relto,$(topsrcdir),$(sort $(mod.$*.files) $(wildcard $(topsrcdir)/build-aux/Makefile.*/??-$*.mk))))
+	@echo   'Documentation :'
+	@printf '%s\n' $(call quote.shell,$(value mod.$*.doc)) | sed -e 's/^# /  /' -e 's/^#//'
 
 $(outdir)/at-noop:
 .PHONY:       $(outdir)/at-noop
diff --git a/build-aux/Makefile.once.head/00-gitfiles.mk b/build-aux/Makefile.once.head/00-gitfiles.mk
index b17b63a..faae91d 100644
--- a/build-aux/Makefile.once.head/00-gitfiles.mk
+++ b/build-aux/Makefile.once.head/00-gitfiles.mk
@@ -15,6 +15,47 @@
 
 mod.gitfiles.description = Automatically populate files.src.src from git
 mod.gitfiles.depends += files nested write-ifchanged quote
+mod.gitfiles.files += $(topsrcdir)/$(gitfiles.file)
+define mod.gitfiles.doc
+# Inputs:
+#   - Global variable    : `gitfiles.file` (Default: gitfiles.mk)
+#   - Directory variable : `nested.subdirs`
+#   - External           : git
+# Outputs:
+#   - File               : `$(topsrcdir)/$(gitfiles.file)`
+#   - Directory variable : `files.src.src`
+#   - Directory variable : `files.src.gen` (only in top dir)
+#
+# The `files` module has a variable (`files.src.src`) that you (the
+# developer) set to list "pure" source files; the type of files that you
+# would check into a version control system.  Since you are a
+# responsible developer, you use a version control system.  Since the
+# computer is already maintaining a list of these files *in the VCS*,
+# why should you--a filthy human--need to also maintain the list?  Enter
+# gitfiles, which will talk to git to maintain `files.src.src`, but
+# won't require that the git repository be distributed to
+# installing-users.
+#
+# If `$(topsrcdir)/.git` exists, then it will generate
+# `$(topsrcdir)/$(gitfiles.file)`.  Otherwise, it will assume that
+# `$(topsrcdir)/$(gitfiles.file)` already exists.
+#
+# It will use the information in `$(topsrcdir)/$(gitfiles.file)` to
+# append to `files.src.src` in each directory
+#
+# Finally, since the generated `$(topsrcdir)/$(gitfiles.file)` must be
+# distributed to users, it is added to $(topsrcdirs)'s `files.src.gen`.
+#
+# When setting `files.src.src`, it needs to know which files "belong" to
+# the current directory directly, and which "belong" to a further
+# subdirectory.  To do this, it uses an expression involving
+# `$(nested.subdirs)`.
+#
+# While gitfiles sets `files.src.src` very early
+# in `each.head`, because `nested.subdirs` might not be set yet, it may
+# or may not be safe to use the value of `$(files.src.src)` in your
+# Makefile, depending on how you set `nested.subdirs`.
+endef
 
 gitfiles.file ?= gitfiles.mk
 
diff --git a/build-aux/Makefile.once.head/00-quote.mk b/build-aux/Makefile.once.head/00-quote.mk
index 9fce401..4c954b1 100644
--- a/build-aux/Makefile.once.head/00-quote.mk
+++ b/build-aux/Makefile.once.head/00-quote.mk
@@ -14,6 +14,24 @@
 # 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
 
 _quote.backslash = $(if $1,$(call _quote.backslash,$(wordlist 2,$(words $1),$1),$(subst $(firstword $1),\$(firstword $1),$2)),$2)
 
@@ -24,6 +42,7 @@ 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.
+# 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)')
diff --git a/build-aux/Makefile.once.head/00-var.mk b/build-aux/Makefile.once.head/00-var.mk
index d1e537e..636bbb0 100644
--- a/build-aux/Makefile.once.head/00-var.mk
+++ b/build-aux/Makefile.once.head/00-var.mk
@@ -14,5 +14,19 @@
 # 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
 
 .PHONY: _var.FORCE
diff --git a/build-aux/Makefile.once.head/10-dist.mk b/build-aux/Makefile.once.head/10-dist.mk
index e139096..8c68d04 100644
--- a/build-aux/Makefile.once.head/10-dist.mk
+++ b/build-aux/Makefile.once.head/10-dist.mk
@@ -14,6 +14,40 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 mod.dist.description = `dist` target for distribution tarballs
+define mod.dist.doc
+# User variables:
+#   - `CP      ?= cp`
+#   - `GZIP    ?= gzip`
+#   - `MKDIR   ?= mkdir`
+#   - `MKDIR_P ?= mkdir -p`
+#   - `MV      ?= mv`
+#   - `RM      ?= rm -f`
+#   - `TAR     ?= tar`
+#   - `GZIPFLAGS ?= $(GZIP_ENV)`
+#   - `GZIP_ENV ?= --best` (only used via `GZIPFLAGS`, not directly)
+# Inputs:
+#   - Global variable    : `dist.exts`    (Default: `.tar.gz`)
+#   - Global variable    : `dist.pkgname` (Default: first of PACKAGE_TARNAME PACKAGE PACKAGE_NAME)
+#   - Global variable    : `dist.version` (Default: first of PACKAGE_VERSION VERSION)
+#   - Directory variable : `files.src`
+# Outputs:
+#   - Directory variable : `files.out.int` (only in top dir)
+#   - .PHONY Target      : `$(outdir)/dist`
+#   - Target             : `$(topoutdir)/$(dist.pkgname)-$(dist.version)`
+#   - Target             : `$(topoutdir)/$(dist.pkgname)-$(dist.version).tar`
+#   - Target             : `$(topoutdir)/$(dist.pkgname)-$(dist.version).tar.gz`
+#
+# Provide the standard `dist` .PHONY target, based on the `files` module
+# information.
+#
+# You may change the default compression target easily via the
+# `dist.exts` variable, but you must define the rule for it manually.
+#
+# Bugs:
+#
+#   The tarball isn't reproducible.  It uses file-system ordering of
+#   files, and includes timestamps.
+endef
 
 # Developer configuration
 
diff --git a/build-aux/Makefile.once.head/10-files.mk b/build-aux/Makefile.once.head/10-files.mk
index 5441735..9d27ae9 100644
--- a/build-aux/Makefile.once.head/10-files.mk
+++ b/build-aux/Makefile.once.head/10-files.mk
@@ -15,6 +15,45 @@
 
 mod.files.description = Keeping track of groups of files
 mod.files.depends += nested
+define mod.files.doc
+# User variables:
+#   - `DESTDIR ?=`
+#   - `RM      ?= rm -f`
+#   - `RMDIR_P ?= rmdir -p --ignore-fail-on-non-empty`
+#   - `TRUE    ?= true`
+# Inputs:
+#   - Global variable    : `files.groups   ?= all`
+#   - Global variable    : `files.default  ?= all`
+#   - Global variable    : `files.vcsclean ?= files.vcsclean`
+#   - Global variable    : `files.generate ?= files.generate`
+#   - Directory variable : `files.src.src`
+#   - Directory variable : `files.src.int`
+#   - Directory variable : `files.src.cfg`
+#   - Directory variable : `files.src.gen`
+#   - Directory variable : `files.out.slow`
+#   - Directory variable : `files.out.int`
+#   - Directory variable : `files.out.cfg`
+#   - Directory variable : `files.out.$(files.groups)` (well, $(addprefix...))
+#   - Directory variable : `files.sys.$(files.groups)` (well, $(addprefix...))
+# Outputs:
+#   - Global variable : `nested.targets`
+#   - Global variable : `at.targets`
+#   - Global variable : `.DEFAULT_GOAL = $(files.default)`
+#   - Creative .PHONY targets:
+#     - `$(outdir)/$(files.generate))`
+#     - `$(addprefix $(outdir)/,$(files.groups))`
+#     - `$(outdir)/installdirs`
+#     - `$(outdir)/install`
+#   - Destructive .PHONY targets:
+#     - `$(outdir)/uninstall`
+#     - `$(outdir)/mostlyclean`
+#     - `$(outdir)/clean`
+#     - `$(outdir)/distclean`
+#     - `$(outdir)/maintainer-clean`
+#     - `$(outdir)/$(files.vcsclean)`
+#
+# TODO: prose documentation
+endef
 
 files.groups ?= all
 files.default ?= all
diff --git a/build-aux/Makefile.once.head/10-gnuconf.mk b/build-aux/Makefile.once.head/10-gnuconf.mk
index c07cfb5..6d641bb 100644
--- a/build-aux/Makefile.once.head/10-gnuconf.mk
+++ b/build-aux/Makefile.once.head/10-gnuconf.mk
@@ -13,10 +13,21 @@
 # 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/>.
 
-# This file is based on §7.2 "Makefile Conventions" of the release of
-# the GNU Coding Standards dated July 25, 2016.
-
 mod.gnuconf.description = GNU standard configuration variables
+define mod.gnuconf.doc
+# Inputs:
+#   - Global variable: `gnuconf.pkgname`
+#      (Default: `$(firstword $(PACKAGE_TARNAME) $(PACKAGE) $(PACKAGE_NAME))`)
+# Outputs:
+#   (see below)
+#
+# This module defines default values (using `?=`) a huge list of
+# variables specified in the GNU Coding Standards that installing-users
+# expect to be able to set.
+#
+# This is based on §7.2 "Makefile Conventions" of the July 25, 2016
+# release of the GNU Coding Standards.
+endef
 
 gnuconf.pkgname ?= $(firstword $(PACKAGE_TARNAME) $(PACKAGE) $(PACKAGE_NAME))
 ifeq ($(gnuconf.pkgname),)
diff --git a/build-aux/Makefile.once.head/10-nested.mk b/build-aux/Makefile.once.head/10-nested.mk
index af9fdf7..72e15ab 100644
--- a/build-aux/Makefile.once.head/10-nested.mk
+++ b/build-aux/Makefile.once.head/10-nested.mk
@@ -14,5 +14,31 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 mod.nested.description = Easy nested .PHONY targets
+define mod.nested.doc
+# Inputs:
+#   - Global variable    : `nested.targets`
+#   - Directory variable : `nested.subdirs`
+# Outputs:
+#   - .PHONY Targets     : `$(addprefix $(outdir)/,$(nested.targets))`
+#   - Variable           : `at.subdirs`
+#
+# The Autothing `at.subdirs` slates a subdirectory's Makefile for inclusion,
+# but doesn't help with recursive targets like `all`, `install`, or `clean`,
+# which one would expect to descend into subdirectories.  Enter `nested`:
+# Define a global list of targets that are recursive/nested, and then in each
+# directory define a list of subdirectries that one would expect them to
+# recurse into.
+#
+# Directories added to `nested.subdirs` are automatically added to `at.subdirs`
+# during the each.tail phase.
+#
+# It may help to think of at.subdirs and nested.subdirs in terms of their
+# Automake conterparts:
+#
+#      | Autothing      | GNU Automake |
+#      +----------------+--------------+
+#      | at.subdirs     | DIST_SUBDIRS |
+#      | nested.subdirs | SUBDIRS      |
+endef
 
 nested.targets ?=
diff --git a/build-aux/Makefile.once.head/10-write-atomic.mk b/build-aux/Makefile.once.head/10-write-atomic.mk
index f099ae2..ce6acd8 100644
--- a/build-aux/Makefile.once.head/10-write-atomic.mk
+++ b/build-aux/Makefile.once.head/10-write-atomic.mk
@@ -1,4 +1,22 @@
 mod.write-atomic.description = `write-atomic` auxiliary build script
 mod.write-atomic.files += $(topsrcdir)/build-aux/write-atomic
+define mod.write-atomic.doc
+# User variables:
+#   - `WRITE_ATOMIC ?= $(topsrcdir)/build-aux/write-atomic`
+# Inputs:
+#   (none)
+# Outputs:
+#   (none)
+#
+# The $(WRITE_ATOMIC) program reads a file from stdin, and writes it to
+# the file named in argv[1], but does so atomically.
+#
+# That is, the following lines are almost equivalient:
+#
+#     ... > $@
+#     ... | $(WRITE_ATOMIC) $@
+#
+# The are only different in that one is atomic, while the other is not.
+endef
 
 WRITE_ATOMIC ?= $(topsrcdir)/build-aux/write-atomic
diff --git a/build-aux/Makefile.once.head/10-write-ifchanged.mk b/build-aux/Makefile.once.head/10-write-ifchanged.mk
index b0a5ac4..5abb3ce 100644
--- a/build-aux/Makefile.once.head/10-write-ifchanged.mk
+++ b/build-aux/Makefile.once.head/10-write-ifchanged.mk
@@ -1,4 +1,22 @@
 mod.write-ifchanged.description = `write-ifchanged` auxiliary build script
 mod.write-ifchanged.files += $(topsrcdir)/build-aux/write-ifchanged
+define mod.write-ifchanged.doc
+# User variables:
+#   - `WRITE_IFCHANGED ?= $(topsrcdir)/build-aux/write-ifchanged`
+# Inputs:
+#   (none)
+# Outputs:
+#   (none)
+#
+# The $(WRITE_IFCHANGED) program reads a file from stdin, and writes it to the
+# file named in argv[1], but does so atomically, but more importantly, does so
+# in a way that does not bump the file's ctime if the new content is the same
+# as the old content.
+#
+# That is, the following lines are almost equivalient:
+#
+#     ... > $@
+#     ... | $(WRITE_ATOMIC) $@
+endef
 
 WRITE_IFCHANGED ?= $(topsrcdir)/build-aux/write-ifchanged
diff --git a/build-aux/Makefile.once.head/zz-mod.mk b/build-aux/Makefile.once.head/zz-mod.mk
index 732f1e1..1b12a2f 100644
--- a/build-aux/Makefile.once.head/zz-mod.mk
+++ b/build-aux/Makefile.once.head/zz-mod.mk
@@ -15,6 +15,28 @@
 
 mod.mod.description = Display information about Autothing modules
 mod.mod.depends += quote
+define mod.mod.doc
+# Inputs:
+#   - Files           : `$(topsrcdir)/build-aux/Makefile.*/??-*.mk`
+#   - Global variable : `mod.*.name`
+#   - Global variable : `mod.*.description`
+#   - Global variable : `mod.*.depends`
+#   - Global variable : `mod.*.files`
+#   - Global variable : `mod.*.doc`
+# Outputs:
+#   - Directory variable : `at.targets`
+#   - .PHONY Target      : `$(outdir)/at-variables-local`
+#   - .PHONY Target      : `$(outdir)/at-variables-global`
+#   - .PHONY Target      : `$(outdir)/at-variables`
+#   - .PHONY Target      : `$(outdir)/at-variables/%`
+#   - .PHONY Target      : `$(outdir)/at-values`
+#   - .PHONY Target      : `$(outdir)/at-values/%`
+#   - .PHONY Target      : `$(outdir)/at-modules`
+#   - .PHONY Target      : `$(outdir)/at-modules/%`
+#   - .PHONY Target      : `$(outdir)/at-noop`
+#
+# TODO: prose documentation
+endef
 
 # The trickery that is _mod.empty/_mod.space is from §6.2 of the GNU Make
 # manual, "The Two Flavors of Variables".
diff --git a/build-aux/Makefile.once.tail/00-dist.mk b/build-aux/Makefile.once.tail/00-dist.mk
index 3fbe0c4..a890d9d 100644
--- a/build-aux/Makefile.once.tail/00-dist.mk
+++ b/build-aux/Makefile.once.tail/00-dist.mk
@@ -21,6 +21,7 @@ $(topoutdir)/$(dist.pkgname)-$(dist.version): $(_dist.files)
 	$(foreach f,$^,$(call _dist.addfile,$(topsrcdir),$(@D)/.tmp.$(@F),$f)$(at.nl))
 	$(MV) $(@D)/.tmp.$(@F) $@ || $(RM) -r $(@D)/.tmp.$(@F)
 
+# TODO: The tar rule isn't reproducible, it should be.
 $(topoutdir)/$(dist.pkgname)-$(dist.version).tar: %.tar: %
 	$(TAR) cf $@ -C $(<D) $(<F)
 $(topoutdir)/$(dist.pkgname)-$(dist.version).tar.gz: %.gz: %