postinst is in btyacc 3.0-5.
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  | #!/bin/sh
set -e
if test $1 != "upgrade"; then
    update-alternatives \
	--install /usr/bin/yacc yacc /usr/bin/btyacc 80 \
        --slave /usr/share/man/man1/yacc.1.gz yaccman /usr/share/man/man1/btyacc.1.gz
fi
exit 0
 |