preinst is in libmono-addins0.2-cil 1.0+git20130406.adcd75b-3.
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 11 12 13 14 15  | #!/bin/sh
set -e
# policy files that were not removed in 0.3~svn.r90520-1 to 0.3-1 upgrade
# see http://bugs.debian.org/547137   
if [ "$1" = "upgrade" ] && dpkg --compare-versions "$2" "<=" "0.3-2"; then
	if [ -f /usr/share/cli-common/packages.d/policy.0.2.Mono.Addins.installcligac ]; then
		echo "Removing forgotten GAC policy file: Mono.Addins 0.2..."
		/usr/share/cli-common/policy-remove Mono.Addins 0.2
	fi
	if [ -f  /usr/share/cli-common/packages.d/policy.0.2.Mono.Addins.Setup.installcligac ]; then
		echo "Removing forgotten GAC policy file: Mono.Addins.Setup 0.2..."
		/usr/share/cli-common/policy-remove Mono.Addins.Setup 0.2
	fi
fi
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