preinst is in gnus 5.11+v0.10.dfsg-3.
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 15 16 | #!/bin/sh
set -e
case "$1" in
  install|upgrade|abort-upgrade)
    # Some old version of this package left a symlink from
    # /etc/xemacs21/site-start.d/20gnus-init.el to
    # /usr/share/emacs/site-lisp/gnus/gnus-init.el, so remove it
    # if it exists. [#339541]
    if [ -L /etc/xemacs21/site-start.d/20gnus-init.el ] &&
       [ "$(readlink -n /etc/xemacs21/site-start.d/20gnus-init.el)" = "/usr/share/emacs/site-lisp/gnus/gnus-init.el" ]; then
        rm -f /etc/xemacs21/site-start.d/20gnus-init.el
    fi
    ;;
esac
 |