The files under /usr, except /usr/local, are not meant to be modified
by hand, but to be installed by the system's package manager.
Admittedly, hand modification of the files in this particular directory
likely wouldn't hurt anything, except in a general way by being
disorderly.  Still, ideally, if you need to modify, then you should
rebuild and reinstall.

The files in this directory are lists.  They are as follows.

dist-root-ctrl.list
    A level-2 list of files to fetch like the Release file.  Such files
    control entire distributions and, thus, order up the various
    per-area control files.  Syntax:

    !FILE       a file to be fetched only for the regular distribution,
                not for backports

per-area-ctrl.list
    A level-3 list of extglob patterns against which the names of
    per-area control files ordered up by dist-root are matched.  If a
    per-area control filename matches no pattern in the list, the file
    is not fetched.  Syntax:

    EXTGLOB     any extglob pattern like, for
                example, +([^/.]) [see bash(1)]
    ${PARAM}    a shell parameter, not necessarily exported, set by the
                program that is using the list:  see the legends at the
                tops of
                    ../scripts/05option-processing.bash
                    ../scripts/07initialization.bash
                and see the whole of
                    /etc/mirrorrib.conf
                for available parameters.
    ${LANGS[*]} any language the user has requested:  this isn't
                literally the shell variable of the same name, but gets
                special handling to recognize the @ language (see the
                manual page)
    PATTERN.@   a pattern that matches a filename with a compression
                extension like, for example, .xz

ctrl-to-decompress.list
    Similar to last list, a level-3 list of extglob patterns against
    which the names of *compressed* per-area control files wanting
    decompression are matched.  The syntax is similar to that of the
    last list, with this addition:

    PATTERN.@X  a pattern that matches a filename with the specific
                compression extension .X

extra-zzz.list
    A backports-only level-3 list of extglob patterns against which the
    names of per-area control files ordered up by dist-root
    are matched.  These are control files that should be indirected
    by hash, only the program cannot deduce the need to do so on
    its own.  Syntax:

    EXTGLOB     as above
    ${PARAM}    as above

nodes-to-prefetch.list
    A level-0 list of symlinks recursively to prefetch because the
    symlinks' referents are too complicated for the program to deduce
    the proper, recursive fetching sequence on its own.  The syntax is
    similar to that of the last list, with this addition:

    :PATTERN    skip for minor version 0 of a Debian release

By "level-n" is meant that n leading directory components are omitted
because implied.  This is best illustrated by example:

    level 0 dists/stable/main/Contents-amd64
    level 1       stable/main/Contents-amd64
    level 2              main/Contents-amd64
    level 3                   Contents-amd64

In all the lists mentioned above (except the level-0 list, which omits
no element), 'dists' is the leading element omitted.

