preinst is in unison2.32.52-gtk 2.32.52-7ubuntu1.
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 17 18  | #!/bin/sh
set -e
case "$1" in
  upgrade)
    if dpkg --compare-versions "$2" lt "2.13.16-1" && dpkg --compare-versions "$2" ge "2.10.2-1"; then
      # This file is common between unison and unison-gtk package
      if true; then
        echo "Deleting alternatives to unison2.10.2-gtk"
        update-alternatives --remove unison-gtk /usr/bin/unison2.10.2-gtk
      else
        echo "Deleting alternatives to unison2.10.2"
        update-alternatives --remove unison /usr/bin/unison2.10.2
      fi
    fi
  ;;
esac
 |