prerm is in mono-runtime 4.6.2.7+dfsg-1ubuntu1.
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 -e
if [ "$1" = remove ]; then
update-alternatives --remove cli /usr/bin/mono
fi
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