prerm is in ubuntu-extras-keyring 2010.09.27.
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 22 23 24 25 26 27 28 | #!/bin/sh
set -e
EXTRAS_KEY="3E5C1192"
case "$1" in
    remove|purge)
	if [ -x /usr/bin/apt-key ]; then
	    apt-key del "$EXTRAS_KEY"
	fi
    ;;
    upgrade|deconfigure)
    ;;
    failed-upgrade)
    ;;
    *)
        echo "prerm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
exit 0
 |