postinst is in freemat 4.2+dfsg1-6.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
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  | #!/bin/sh
set -e
# Move a conffile without triggering a dpkg question
mv_conffile() {
    local OLDCONFFILE="$1"
    local NEWCONFFILE="$2"
    [ -e "$OLDCONFFILE" ] || return 0
    echo "Preserving user changes to $NEWCONFFILE ..."
    mv -f "$NEWCONFFILE" "$NEWCONFFILE".dpkg-new
    mv -f "$OLDCONFFILE" "$NEWCONFFILE"
}
case "$1" in
configure)
    if dpkg --compare-versions "$2" le "4.2+dfsg1-3"; then
        mv_conffile "/etc/xdg/FreeMat/FreeMat v4.0.conf" "/etc/xdg/FreeMat/FreeMat v4.2.conf"
    fi
esac
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