postinst is in guile-2.0-doc 2.0.13+1-5build2.
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 30 31 | #!/bin/sh
set -e
if [ "$1" != upgrade ] ; then
    update-alternatives \
        --install /usr/share/info/guile.info.gz \
        guile.info.gz \
        /usr/share/info/guile-2.0/guile.info.gz \
        2011 \
        --slave /usr/share/info/guile.info-1.gz guile.info-1.gz \
                /usr/share/info/guile-2.0/guile.info-1.gz \
        --slave /usr/share/info/guile.info-2.gz guile.info-2.gz \
                /usr/share/info/guile-2.0/guile.info-2.gz \
        --slave /usr/share/info/guile.info-3.gz guile.info-3.gz \
                /usr/share/info/guile-2.0/guile.info-3.gz \
        --slave /usr/share/info/guile.info-4.gz guile.info-4.gz \
                /usr/share/info/guile-2.0/guile.info-4.gz \
        --slave /usr/share/info/guile.info-5.gz guile.info-5.gz \
                /usr/share/info/guile-2.0/guile.info-5.gz \
        --slave /usr/share/info/guile.info-6.gz guile.info-6.gz \
                /usr/share/info/guile-2.0/guile.info-6.gz \
        --slave /usr/share/info/guile.info-7.gz guile.info-7.gz \
                /usr/share/info/guile-2.0/guile.info-7.gz \
        --slave /usr/share/info/guile.info-8.gz guile.info-8.gz \
                /usr/share/info/guile-2.0/guile.info-8.gz \
        --slave /usr/share/info/guile.info-9.gz guile.info-9.gz \
                /usr/share/info/guile-2.0/guile.info-9.gz \
        --slave /usr/share/info/guile.info-10.gz guile.info-10.gz \
                /usr/share/info/guile-2.0/guile.info-10.gz
fi
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