prerm is in lzma 9.22-2ubuntu2.
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 | #!/bin/sh
set -e
if
	test "$1" != upgrade ||
	# downgrading to a pre-alternatives version
	dpkg --compare-versions "$2" lt-nl 9.22-1
then
	update-alternatives --remove lzma /usr/bin/lzmp
fi
 |