prerm is in mono-devel 3.2.8+dfsg-4ubuntu1.
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 | #!/bin/sh -e
if [ "$1" = remove ]; then
        update-alternatives --remove c-sharp-compiler		/usr/bin/mono-csc
        update-alternatives --remove resource-file-generator	/usr/bin/resgen
        update-alternatives --remove assembly-linker		/usr/bin/al
        update-alternatives --remove strong-name-tool		/usr/bin/sn
fi
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