This file is indexed.

prerm is in gnuplot-qt 5.0.5+dfsg1-6+deb9u1.

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.

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#!/bin/sh

set -e

if [ "$1" != "upgrade" ]; then
    update-alternatives --remove gnuplot /usr/bin/gnuplot-qt
    update-alternatives --remove gnuplot5 /usr/bin/gnuplot-qt
    update-alternatives --remove gnuplot /usr/bin/gnuplot4-qt
    update-alternatives --remove gnuplot4 /usr/bin/gnuplot4-qt
fi