/usr/share/ltsp/plugins/ltsp-build-client/Debian-functions is in ltsp-server 5.3.7-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | # functions specific to Debian (and possibly derived distros)
add_mirrors() {
    # feed a list of comma-separated or enter-separated mirrors,
    # add them to the chroot's sources.list
    echo "$1" | tr ',' '\n' \
    | while read mirror dist components; do
        if [ -z "$mirror" ] || [ "$mirror" = "none" ]; then
            continue
        fi
        if [ -z "$dist" ]; then
            dist="$DIST"
            # components are only optional if dist does not end with a trailing
            # slash; always include them if dist is not specified.
            components="$COMPONENTS"
        fi
        echo "deb $mirror $dist $components" >> $ROOT/etc/apt/sources.list
        case $mirror in
            file:///*) dir=$(echo "$mirror" | sed -e 's,^file://,,g')
                mkdir -p $ROOT/$dir
                chroot_mount $dir $dir --bind
                ;;
        esac
    done
}
add_mirror() {
    # compatibility function
    echo "WARNING: use of deprecated add_mirror or add_multiple_mirrors"
    echo "function. please update your ltsp-build-client plugins."
    add_mirrors "$@"
}
add_multiple_mirrors() {
   add_mirror "$@" 
}
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