postinst is in chezscheme 9.5+dfsg-2build2.
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 | #!/bin/sh
set -e
if [ "$1" = "configure" ]; then
    update-alternatives \
        --install /usr/bin/scheme-script scheme-script /usr/bin/chezscheme9.5 10 \
        --slave /usr/share/man/man1/scheme-script.1.gz scheme-script.1.gz \
                /usr/share/man/man1/scheme-script9.5.1.gz
fi
 |