preinst is in bison 1:2.5.dfsg-2.1.
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 | #!/bin/bash
# On error, bomb out
set -e
# clean error from previous package.
rm -f /usr/share/man/man1/bison.1
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