prerm is in valac-0.14 0.14.2-0ubuntu1.
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 | #!/bin/sh
set -e
update-alternatives --remove \
valac /usr/bin/valac-0.14
|