postinst is in lzma 9.22-2.
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 | #!/bin/sh
set -e
update-alternatives \
	--install /usr/bin/lzma lzma /usr/bin/lzmp 30 \
	--slave /usr/bin/unlzma unlzma /usr/bin/lzmp \
	--slave /usr/bin/lzcat lzcat /usr/bin/lzmp \
	--slave /usr/share/man/man1/lzma.1.gz lzma.1.gz \
		/usr/share/man/man1/lzmp.1.gz \
	--slave /usr/share/man/man1/unlzma.1.gz unlzma.1.gz \
		/usr/share/man/man1/lzmp.1.gz \
	--slave /usr/share/man/man1/lzcat.1.gz lzcat.1.gz \
		/usr/share/man/man1/lzmp.1.gz
 |