postinst is in guile-2.0-libs 2.0.13+1-4.
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 19 20 21 22 23 24 25 26 27 28 29 | #!/bin/sh
set -e
case "$1" in
  configure)
    if test -d /usr/share/slib; then
      dpkg-trigger /usr/share/slib
    fi
    ;;
  triggered)
    case " $2 " in
      *" /usr/share/slib "*)
        # Configure slib support.
        if test -d /usr/share/slib
        then
          # Uncomment the code below whenever SLIB supports Guile 2.0.
          echo "SLIB does not support Guile 2.0 yet.  Ignoring."
          #(cd /usr/share/guile/2.0 && ln -sf ../../slib .)
          #/usr/lib/guile-2.0/bin/guile -c \
          #  "(use-modules (ice-9 slib)) (require 'new-catalog)"
        else
        # slib isn't installed -- clean up symlink.
          rm -f /usr/share/guile/2.0/slib
        fi
        ;;
    esac
    ;;
esac
 |