postrm is in apt-listdifferences 1.20170813.
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 | #!/bin/sh
set -e
. /usr/share/debconf/confmodule
if [ "$1" = "purge" ]; then
    db_input high apt-listdifferences/purge || true
    db_go
    db_get apt-listdifferences/purge
    test "$RET" = "false" || rm -rf /var/cache/apt/sources
fi
# Automatically added by dh_installdebconf/10.7.2ubuntu2
if [ "$1" = purge ] && [ -e /usr/share/debconf/confmodule ]; then
	. /usr/share/debconf/confmodule
	db_purge
fi
# End automatically added section
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