prerm is in mate-session-manager 1.12.2-1.
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 | #!/bin/sh
set -e
if [ "$1" = "remove" ]; then
update-alternatives --remove x-session-manager /usr/bin/mate-session
fi
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