prerm is in unison2.32.52-gtk 2.32.52-7ubuntu1.
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  | #!/bin/sh
set -e
if [ "$1" = "remove" ]; then
    update-alternatives --remove unison /usr/bin/unison-2.32.52-gtk
    if true; then
        update-alternatives --remove unison-gtk /usr/bin/unison-2.32.52-gtk
    fi
fi
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