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+(This file was generated by makeinfo and splitinfo.gawk.)
+(Released under the old-style GNU documentation license;
+ see sources or other output files for full text.)
+
+6 Layout and infrastructure
+***************************
+
+The TeX Live source tree is the subtree rooted at 'Build/source' of the
+complete TL distribution and contains the sources for all executables
+distributed by TL, as well as 'configure' scripts and 'make' rules to
+build and install them together with some of their support files.
+
+6.1 Build system tools
+======================
+
+As mentioned above (*note Prerequisites::), a normal build has few
+requirements.  On the other hand, if you want to modify the TeX Live
+infrastructure sources, such as 'configure.ac' or 'Makefile.am' files,
+you will need to have several additional tools installed.
+
+   In general, the TL build system uses the latest released versions of
+the GNU build tools, installed directly from the original GNU releases
+(e.g., by building them with 'configure --prefix=/usr/local/gnu' and
+having 'PATH' start with '/usr/local/gnu/bin').  We have found that
+trying to use the versions of these tools packaged for distros causes
+many extra hassles, so don't do that, tempting as it may be.
+
+   Currently the versions we use are:
+
+     autoconf (GNU Autoconf) 2.69
+     automake (GNU automake) 1.16.1
+     bison (GNU Bison) 3.5.2
+     flex 2.6.0
+     ltmain.sh (GNU libtool) 2.4.6
+     m4 (GNU M4) 1.4.18
+     makeinfo (GNU texinfo) 6.7
+
+   These versions should be used to update the generated files (e.g.,
+'configure' or 'Makefile.in') in all or parts of the TL tree after their
+dependencies have been changed.  This can be done explicitly with the
+top-level 'reautoconf' script or implicitly by using the configure
+option '--enable-maintainer-mode'.
+
+   It has often turned out that the bison and flex versions are not
+critical; however, the autotools versions are.  If you don't have the
+given versions, get them before modifying the build infrastructure.
+
+   The files in the Subversion repository (see
+<https://tug.org/texlive/svn>) are all up to date (barring bugs).  For
+this to be reflected by their timestamps in your checkout, be sure to
+set 'use-commit-times=yes' in '~/.subversion/config' or the equivalent.
+
+   If timestamps are wrong, you may also be able to avoid unnecessary
+runs of 'bison', 'flex', or 'makeinfo' with 'touch' of the generated
+('.c', '.h', or '.info') files.  With '--enable-maintainer-mode' it may
+also be necessary to 'touch' first 'aclocal.m4', then 'configure' and
+'config.h.in' (or 'c-auto.in'), and finally all 'Makefile.in' files.
+
+6.2 Top-level directories
+=========================
+
+Here is a brief description of the top-level directories in the TeX Live
+source tree.
+
+   As mentioned at the beginning of *note Overview of build system::,
+the main source directories are 'texk/' (TeX-specific programs and
+libraries), 'utils/' (additional programs), and 'libs/' (generic
+libraries).
+
+   In addition, the top-level directories 'am/' and 'm4/' contain
+'Makefile.am' fragments and Autoconf macros, respectively, used in many
+places.  Specifically, the file 'm4/kpse-pkgs.m4' contains lists of all
+program and library modules; missing modules are silently ignored.
+(This helps in creating cut-down source trees.)
+
+   Each module contributes fragments (in separate files) defining its
+capabilities and requirements to the 'configure.ac' scripts at the
+top-level and in the subdirectories 'libs', 'utils', and 'texk'.  The
+fragments from program modules supply 'configure' options to disable or
+enable building them; those from library modules specify if an installed
+(system) version of that library can be used.  This ultimately
+determines which modules need to be built--although all modules must be
+configured for the benefit of 'make' targets such as 'dist' or
+'distcheck'.
+
+   The top-level 'build-aux/' directory contains the common files
+'compile', 'config.guess', 'config.sub', 'depcomp', etc. used by most
+packages.  These are taken from the GNU Gnulib sources
+(<https://www.gnu.org/software/gnulib>), which in turn synchronizes with
+any ultimate upstream repository.  There are independent copies of some
+of these in a few other places, e.g.,
+'libs/freetype2/freetype-*/builds/unix/'.  The 'reautoconf' script does
+not touch those, but a TL cron job keeps them in sync (nightly).
+
+   When the top-level './Build' script is used to build TL, two more two
+more top-level directories appear: 'Work/' for the build tree, and
+'inst/' for the install tree (from 'make install').  These names (and
+everything else about 'Build''s operation) can be changed by setting
+environment variables before running it; see the script source.
+
+6.3 Autoconf macros
+===================
+
+Here we describe a few of the Autoconf macros used in several
+modules--many more are defined in the sources; see the top-level 'm4/'
+directory.  These general macros are supplemented by module-specific
+macros in directories such as 'texk/dvipng/m4/'; some of those are
+described in following sections (*note Library modules:: and *note
+Program modules::).
+
+6.3.1 General setup macros
+--------------------------
+
+The TL sources use two general setup macros:
+
+ -- Macro: KPSE_BASIC (NAME, [MORE-OPTIONS])
+     Initialize the basic TL infrastructure for module NAME:
+        'AM_INIT_AUTOMAKE([foreign MORE-OPTIONS])'
+        'AM_MAINTAINER_MODE'
+        'KPSE_COMPILER_WARNINGS'
+     and make sure the C compiler understands function prototypes.  This
+     is used for all generic library and program modules.
+
+ -- Macro: KPSE_COMMON (NAME, [MORE-OPTIONS])
+     Like 'KPSE_BASIC' but add:
+        'LT_PREREQ([2.2.6])'
+        'LT_INIT([win32-dll])'
+        'AC_SYS_LARGEFILE'
+        'AC_FUNC_FSEEKO'
+     along with checks for frequently used functions, headers, types,
+     and structures.  This is used for TeX-specific modules.
+
+6.3.2 Macros for programs
+-------------------------
+
+Macros for program checks:
+
+ -- Macro: KPSE_CHECK_LATEX
+     Set 'LATEX' to the first of 'latex', 'elatex', or 'lambda' which
+     exists in 'PATH', or to 'no' if none of them exists.  Call
+     'AC_SUBST' for 'LATEX'.  The result of this test can be overridden
+     by setting the 'LATEX' environment variable or the cache variable
+     'ac_cv_prog_LATEX'.
+
+ -- Macro: KPSE_CHECK_PDFLATEX
+     Check for 'pdflatex' in 'PATH' and set 'PDFLATEX'.
+
+ -- Macro: KPSE_CHECK_PERL
+     Check for 'perl' or 'perl5' in 'PATH' and set 'PERL'.
+
+ -- Macro: KPSE_PROG_LEX
+     Call 'AC_PROG_LEX' and add the flag '-l' for 'flex'.
+
+6.3.3 Macros for compilers
+--------------------------
+
+Macros for compiler-related checks:
+
+ -- Macro: KPSE_COMPILER_WARNINGS
+     When using the (Objective) C/C++ compiler, set
+     'WARNING_[OBJ]C[XX]FLAGS' to suitable warning flags (depending on
+     the value given to or implied for '--enable-compiler-warnings').
+     Call 'AC_SUBST' for them.  At present this assumes GNU compiler
+     warning options, but could be extended to others if necessary.
+
+     This macro caches its results in the 'kpse_cv_warning_cflags', ...
+     variables.
+
+ -- Macro: KPSE_COMPILER_VISIBILITY
+     When using the C or C++ compiler, try to set
+     'VISIBILITY_C[XX]FLAGS' to flags to hide external symbols.  Call
+     'AC_SUBST' for this variable.  At present this only tests for the
+     compiler option '-fvisibility=hidden', but could be extended if
+     necessary.
+
+     This macro caches its results in the 'kpse_cv_visibility_cflags' or
+     'kpse_cv_visibility_cxxflags' variable.
+
+ -- Macro: KPSE_CXX_HACK
+     Provide the configure option '--enable-cxx-runtime-hack'.  If
+     enabled and when using 'g++', try to statically link with
+     'libstdc++', notably improving portability of the resulting binary.
+
+     This macro caches its result in the 'kpse_cv_cxx_hack' variable.
+
+6.3.4 Macros for libraries
+--------------------------
+
+One macro for a library check:
+
+ -- Macro: KPSE_LARGEFILE (VARIABLE, [EXTRA-DEFINE])
+     Call 'AC_SYS_LARGEFILE' and 'AC_FUNC_FSEEKO' and append suitable
+     '-D' flags (optionally including '-DEXTRA-DEFINE') to VARIABLE.
+
+6.3.5 Macros for library and header flags
+-----------------------------------------
+
+Each library module 'libs/LIB' or 'texk/LIB' is supplemented by a macro
+'KPSE_LIB_FLAGS' (all uppercase) that provides make variables for that
+library.  E.g., for 'libs/libpng':
+
+ -- Macro: KPSE_LIBPNG_FLAGS
+     Provide the configure option '--with-system-libpng'.  Set and
+     'AC_SUBST' 'make' variables for modules using this library (either
+     an installed version or from the TeX Live tree):
+
+     'LIBPNG_INCLUDES'   for use in 'CPPFLAGS',
+     'LIBPNG_LIBS'       for use in 'LDADD',
+     'LIBPNG_DEPEND'     for use as a Makefile dependency,
+     'LIBPNG_RULE'       for the 'make' rules to rebuild the library.
+
+ -- Macro: KPSE_ADD_FLAGS (NAME)
+     Temporarily extend 'CPPFLAGS' and 'LIBS' with the values required
+     for the library module 'NAME'.
+
+ -- Macro: KPSE_RESTORE_FLAGS
+     Restore 'CPPFLAGS' and 'LIBS' to their original values.
+
+   As an example, the 'configure.ac' file for a hypothetical program
+'utils/foo' using 'libpng', and hence 'zlib', would contain
+     KPSE_ZLIB_FLAGS
+     KPSE_LIBPNG_FLAGS
+
+and its 'Makefile.am' would be along these lines:
+     bin_PROGRAMS = foo
+     AM_CPPFLAGS = ${LIBPNG_INCLUDES} ${ZLIB_INCLUDES}
+     foo_LDADD = ${LIBPNG_LIBS} ${ZLIB_LIBS}
+     foo_DEPENDENCIES = ${ZLIB_DEPEND} ${LIBPNG_DEPEND}
+     ## Rebuild libz
+     @ZLIB_RULE@
+     ## Rebuild libpng
+     @LIBPNG_RULE@
+
+   If it were necessary to examine whether certain 'zlib' or 'libpng'
+features were available, 'configure.ac' should be continued this way:
+     KPSE_ADD_FLAGS([zlib])
+     ... # tests for 'zlib' features, if any
+     KPSE_ADD_FLAGS([libpng])
+     ... # tests for 'libpng' features
+     KPSE_RESTORE_FLAGS # restore 'CPPFLAGS' and 'LIBS'
+
+6.3.6 Macros for Windows
+------------------------
+
+Windows differs in several aspects from Unix-like systems, many of them
+due to the lack of symbolic links.
+
+ -- Macro: KPSE_CHECK_WIN32
+     Check if compiling for a Windows system.  The result is either 'no'
+     for Unix-like systems (including Cygwin), 'mingw32' for Windows
+     with GCC, or 'native' for Windows with MSVC. The result is cached
+     in the 'kpse_cv_have_win32' variable.
+
+ -- Macro: KPSE_COND_WIN32
+     Call 'KPSE_CHECK_WIN32' and define the Automake conditional 'WIN32'
+     ('true' if the value of 'kpse_cv_have_win32' is not 'no').
+
+ -- Macro: KPSE_COND_MINGW32
+     Call 'KPSE_COND_WIN32' and define the Automake conditional
+     'MINGW32' ('true' if the value of 'kpse_cv_have_win32' is
+     'mingw32').
+
+ -- Macro: KPSE_COND_WIN32_WRAP
+     Call 'KPSE_COND_WIN32' and define the Automake conditional
+     'WIN32_WRAP' ('true' if the standard Windows wrapper
+     ('texk/texlive/w32_wrapper/runscript.exe') exists.  This wrapper is
+     used on Windows instead of symlinks for the "linked scripts" (*note
+     Linked scripts::).
+
+ -- Macro: KPSE_WIN32_CALL
+     Call 'KPSE_COND_WIN32' and check if the file
+     'texk/texlive/w32_wrapper/callexe.c' exists; if it does, create a
+     symlink in the build tree.  Compiling 'callexe.c' with
+     '-DEXEPROG='"FOO.exe"'' and installing 'callexe.exe' as 'BAR.exe'
+     is used on Windows instead of a symlink 'BAR->FOO' for Unix-like
+     systems.
+
+6.4 Library modules
+===================
+
+Here we discuss some specifics for a few of the libraries in TL, both
+for the details themselves, and as a way of illuminating the general
+structure and variation.
+
+6.4.1 The 'png' library in 'libs/libpng'
+----------------------------------------
+
+The "generic" 'png' library uses the source tree in the subdirectory
+'libpng-src/', with all modifications for TL recorded in 'TLpatches/*'.
+The 'configure.ac' fragment 'ac/withenable.ac' contains
+
+     KPSE_WITH_LIB([libpng], [zlib])
+
+to specify the module name and indicate the dependency on 'zlib'.  A
+third literal argument ''tree'' would specify that the library from the
+TeX Live tree cannot be replaced by a system version.  That not being
+the case here, a second fragment 'ac/libpng.ac' contains
+
+     KPSE_TRY_LIB([libpng],
+                  [#include <png.h>],
+                  [png_structp png; png_voidp io; png_rw_ptr fn;
+     png_set_read_fn(png, io, fn);])
+
+thus providing the simple C code
+
+     #include <png.h>
+     int main ()
+     { png_structp png; png_voidp io; png_rw_ptr fn;
+       png_set_read_fn(png, io, fn);
+       return 0; }
+
+which Autoconf uses to verify the usability of a system version with C
+code.  The analogous macro 'KPSE_TRY_LIBXX' would check using C++.
+These fragments are included by the 'configure.ac' at the top level of
+TL ('Build/source/configure.ac').
+
+   For this library, like many other modules, a proxy build system for
+TL is used, consisting of our own 'configure.ac', 'Makefile.am',
+'include/Makefile.am'; the distributed build system is not used.
+(Consequently, a few generated files and auxiliary scripts are removed
+from the distributed source tree.)
+
+   The public headers 'png.h', 'pngconf.h', and 'pnglibconf.h' are
+"installed" (as symlinks) under 'include/' in the build tree exactly as
+they are for a system version under, e.g., '/usr/include/'.
+
+   The module is supplemented by the file 'm4/kpse-libpng-flags.m4' that
+defines the M4 macro 'KPSE_LIBPNG_FLAGS' used by all modules depending
+on this library in their 'configure.ac' to generate the 'make' variables
+'LIBPNG_INCLUDES' for use in 'CPPFLAGS', 'LIBPNG_LIBS' for use in
+'LDADD', 'LIBPNG_DEPEND' for use as dependencies, and 'LIBPNG_RULE' for
+the 'make' rules to rebuild the library.
+
+   'm4/kpse-libpng-flags.m4' also supplies the configure option
+'--with-system-libpng', which then uses 'pkg-config' to determine the
+flags required for the system library.
+
+6.4.2 The 'zlib' library in 'libs/zlib'
+---------------------------------------
+
+This generic library is very much analogous to 'libpng', but without the
+dependency on any other library.  The file 'm4/kpse-zlib-flags.m4'
+supplies the configure option '--with-system-zlib', as well as
+'--with-zlib-includes' and '--with-zlib-libdir' to specify non-standard
+locations of the 'zlib' headers and/or library.
+
+6.4.3 The 'freetype' library in 'libs/freetype2'
+------------------------------------------------
+
+This module uses a wrapper build system.  In contrast to the proxy build
+described earlier, the wrapper build has an almost trivial
+'configure.ac' and a 'Makefile.am' which invokes the 'configure' and
+'make' in the distributed source, followed by 'make install' with the TL
+build tree as destination.  In other words, this actually uses the build
+system provided by upstream (possibly patched).
+
+   The flags required for the system library are obtained through
+'freetype-config'.
+
+6.4.4 The 'kpathsea' library in 'texk/kpathsea'
+-----------------------------------------------
+
+This is one of the TeX-specific libraries that are maintained as part of
+TeX Live (*note (kpathsea)::); the other is 'ptexenc'.  These TeX
+libraries are Libtool libraries (static and/or shared) and are installed
+by 'make install' together with the programs.  They are, however, not
+part of the TL DVD as distributed by TeX user groups, and have never
+been officially released for standalone use.
+
+   It is possible, and probably useful for distro builds (*note Distro
+builds::), to specify the configure option '--with-system-kpathsea' in
+order to use a system version of the library.  Programs outside the TL
+tree should use 'pkg-config' for the required flags.
+
+   In addition to 'kpathsea/ac/withenable.ac' and
+'kpathsea/ac/kpathsea.ac' here there is a third fragment
+'kpathsea.ac/mktex.ac', included by both 'withenable.ac' and
+'configure.ac', which supplies configure options such as
+'--enable-mktextfm-default'.  These determine the compile time default
+of whether or not to run 'mktextfm' (and similar) to generate a missing
+'.tfm' (or whatever) file.  In any case, however, the command line
+options '-mktex=tfm' or '-no-mktex=tfm' for the TeX-like engines
+override this default.
+
+6.5 Program modules
+===================
+
+As with libraries (*note Library modules::), here we discuss the details
+for a few of the programs in TL.
+
+6.5.1 The 't1utils' package in 'utils/t1utils'
+----------------------------------------------
+
+Here we use the distributed source tree 't1utils-src' with modifications
+documented in 'TLpatches/*' and a proxy build system consisting of
+'configure.ac' and 'Makefile.am'.  The fragment 'ac/withenable.ac'
+contains
+
+     KPSE_ENABLE_PROG([t1utils])
+
+specifying the module name without any dependencies, and supplies the
+configure option '--disable-t1utils'.
+
+6.5.2 The 'xindy' package in 'utils/xindy'
+------------------------------------------
+
+This module uses the distributed source tree 'xindy-src/' with
+modifications documented in 'TLpatches/*', and a wrapper 'configure.ac'
+and 'Makefile.am' that descends into 'xindy-src'.
+
+   The 'xindy' build requires a 'make' that supports a 'VPATH' build,
+can handle all targets, and do not refer to '${top_srcdir}' or
+'${top_builddir}'.  The fragment 'xindy/ac/withenable.ac' contains
+
+     KPSE_ENABLE_PROG([xindy], , [disable])
+     m4_include(kpse_TL[utils/xindy/ac/xindy.ac])
+     m4_include(kpse_TL[utils/xindy/ac/clisp.ac])
+
+where 'disable' in the third argument indicates that 'xindy' is only
+built if explicitly enabled by the user with 'configure --enable-xindy'
+(the need for 'clisp' makes it too painful to enable by default).
+
+   The additional fragments 'ac/xindy.ac' and 'ac/clisp.ac' specify more
+'configure' options to be seen at the top level, with 'ac/xindy.ac' also
+included by 'configure.ac'.
+
+6.5.3 The 'xdvik' package in 'texk/xdvik'
+-----------------------------------------
+
+This package is maintained as part of the TeX Live tree with sources in
+its own directory ('texk/xdvik/').  The fragment
+'xdvik/ac/withenable.ac' contains
+
+     dnl extra_dirs = texk/xdvik/squeeze
+     KPSE_ENABLE_PROG([xdvik], [kpathsea freetype2], [x])
+     m4_include(kpse_TL[texk/xdvik/ac/xdvik.ac])
+
+thus specifying dependencies on the 'kpathsea', 'freetype', and X11
+libraries.  The M4 comment (following 'dnl') signals the subsidiary
+'squeeze/configure.ac'.  This is needed because the main executable
+'xdvi-bin' (to be installed as, e.g., 'xdvi-xaw') is for the 'host'
+system whereas the auxiliary program 'squeeze/squeeze' has to run on the
+'build' system; in a cross compilation, these differ.
+
+   The additional fragment 'ac/xdvik.ac' is also included by
+'configure.ac' and supplies the configure option '--with-xdvi-x-toolkit'
+also seen at the top level.
+
+6.5.4 The subdirectory 'utils/asymptote'
+----------------------------------------
+
+This subdirectory contains the sources for 'asy' and 'xasy' but due to
+its complexity and prerequisites (e.g., OpenGL) it is not part of the TL
+build system.  These programs must be built and installed independently,
+but are included on the TL DVD together with their support files.  See
+<https://tug.org/build.html#asymptote>.
+
+6.6 Extending TeX Live
+======================
+
+This section outlines the basic process for adding new packages to the
+TL build system.
+
+   In any case, a new package directory 'foo' should contain the
+original sources, modified only with changes necessary for TL, in
+'foo/foo-src'.  The changes should be documented in 'foo/TLpatches/*',
+and also be submitted upstream whenever reasonable.  In addition, 'foo/'
+will need the usual Automake build-related files ('configure.ac',
+'Makefile.am', etc.  Please maintain 'foo/ChangeLog' for all TL changes.
+
+6.6.1 Adding a new program module
+---------------------------------
+
+A TeX-specific program module in a subdirectory 'texk/PROG' may use the
+TeX-specific libraries and is included by adding its name 'PROG' to the
+M4 list 'kpse_texk_pkgs' defined in 'm4/kpse-pkgs.m4'.
+
+   A generic program module in a subdirectory 'utils/PROG' must not use
+the TeX-specific libraries and is included by adding its name 'PROG' to
+the M4 list 'kpse_utils_pkgs' in 'm4/kpse-pkgs.m4'.
+
+   In either case, the subdirectory 'texk/PROG' or 'utils/PROG' must
+provide a fragment 'ac/withenable.ac' that contains the M4 macro
+'KPSE_ENABLE_PROG' defined in 'm4/kpse-setup.m4' with 'PROG' as the
+mandatory first argument and three optional arguments:
+
+  1. a list of required libraries from the TL tree;
+
+  2. a list of options: 'disable' if this module is not to be built
+     without the configure option '--enable-PROG', 'native' if cross
+     compilation is not possible, 'x' if the program requires X11
+     libraries;
+
+  3. a comment added to the help text for the 'configure' option
+     '--enable-PROG' or '--disable-PROG'.
+
+   If the module requires specific 'configure' options to be seen at the
+top level, they should be defined in an additional fragment 'ac/PROG.ac'
+included from 'ac/withenable.ac' and 'configure.ac'.
+
+   Usually, the new program is maintained somewhere outside of TeX Live.
+In that case, as above, we put the upstream sources into a subdirectory
+'PROG-src' (e.g., 'utils/newprog/newprog-src').  We do not typically run
+'configure' in this original '...-src' directory, but only in our own
+directory; but we do compile using the source files in '...-src'.
+
+   So, to summarize the files that we must (usually) create:
+
+'ac/withenable.ac'
+     The 'KPSE_ENABLE_PROG' call just explained.
+
+'configure.ac'
+'Makefile.am'
+     By merging the contents of the original 'configure.ac' (if
+     provided) and a comparable program already in TL. There is no magic
+     recipe, it's necessary to think about needs to be done in the
+     original vs. in TeX Live.
+
+'TLpatches/TL-Changes'
+     Actions taken after getting the original source tree; typically
+     removal of derived or unused common files.
+
+'TLpatches/patch-...'
+     If any changes are needed to the original sources, record the
+     patches here so they can be applied next time.  And send them
+     upstream so that perhaps they won't have to be.
+
+'ChangeLog'
+     Record all TL-specific changes, now and in the future.
+
+   Then, run GNU 'autoreconf' in the new directory (*note Build system
+tools::).  After that works, 'svn add' the necessary files, including
+the generated 'Makefile.in aclocal.m4 configure', and 'svn:ignore' the
+Automake cache 'autom4te.cache'.  (This is so people checking out the TL
+source tree do not have to run any autotools, but can simply run
+'configure'.)
+
+   Then, run the TL tool 'reautoconf' in the top-level TL 'source/'
+directory, to incorporate the new program into the build tree.
+
+   Then, run (and rerun) a build until the program compiles and tests
+successfully, probably involving rerunning autoreconf in the source
+directory, 'make' in the build directory, etc.
+
+   After final success, don't forget to commit.
+
+6.6.2 Adding a new generic library module
+-----------------------------------------
+
+A generic library module in a subdirectory 'libs/LIB' must not depend on
+TeX-specific libraries, by definition.  It is included by adding its
+name 'LIB' to the M4 macro 'kpse_libs_pkgs' in 'm4/kpse-pkgs.m4'--before
+any other libraries from the TeX Live tree on which it depends.
+
+   As with program modules, the subdirectory 'libs/LIB' must contain the
+sources and build system for the library (and any installable support
+programs) and a fragment 'ac/withenable.ac' that contains the M4 macro
+'KPSE_WITH_LIB' defined in 'm4/kpse-setup.m4' with 'LIB' as the
+mandatory first argument and two optional arguments: a list of required
+libraries from the TL tree, and a list of options: for libraries,
+currently there is only one--specify 'tree' if this library cannot be
+replaced by a system version.
+
+   If a system version can be used, a second fragment 'ac/LIB.ac' is
+needed, containing the M4 macro 'KPSE_TRY_LIB' (or 'KPSE_TRY_LIBXX')
+with 'LIB' as the mandatory first argument and two additional arguments
+for the Autoconf macro 'AC_LANG_PROGRAM' used to compile and link a
+small C (or C++) program as sanity check for using the system library.
+
+   In addition a file 'm4/kpse-LIB-flags' (at the top level) must define
+the M4 macro 'KPSE_LIB_FLAGS' (all uppercase) setting up the 'make'
+variables 'LIB_INCLUDES', 'LIB_LIBS', 'LIB_DEPEND', and 'LIB_RULE' with
+the values required for 'CPPFLAGS', 'LDADD', dependencies, and a
+(multi-line) 'make' rule to rebuild the library when necessary.  All of
+that is needed for the library from the TL tree and, if supported, for a
+system version.
+
+   If a system library is allowed, 'KPSE_LIB_FLAGS' also provides the
+configure option '--with-system-LIB' and uses the additional M4 macro
+'KPSE_LIB_SYSTEM_FLAGS' to generate the 'make' variables for a system
+library.  In addition, the definition of the M4 macro
+'KPSE_ALL_SYSTEM_FLAGS' in 'm4/kpse-pkgs.m4' must be extended by the
+line:
+   'AC_REQUIRE([KPSE_LIB_SYSTEM_FLAGS])'
+
+6.6.3 Adding a new TeX-specific library module
+----------------------------------------------
+
+A TeX-specific library module in a subdirectory 'texk/LIB' may depend on
+other TeX-specific libraries but must not depend on any generic library
+from the TL tree.  It is included in the same general ways as a generic
+library (see the previous section), with these modifications:
+
+   * The library name 'LIB' is added to the M4 macro
+     'kpse_texlibs_pkgs', which is also in 'm4/kpse-pkgs.m4'.
+
+   * The fragment 'ac/withenable.ac' must use 'KPSE_WITH_TEXLIB'.
+