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diff --git a/lib/Devscripts/Set.pm b/lib/Devscripts/Set.pm
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+# Copyright Bill Allombert <ballombe@debian.org> 2001.
+# Modifications copyright 2002 Julian Gilbey <jdg@debian.org>
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+package Devscripts::Set;
+
+use strict;
+
+BEGIN {
+    use Exporter ();
+    use vars qw(@EXPORT @ISA %EXPORT_TAGS);
+    @EXPORT      = qw(SetMinus SetInter SetUnion);
+    @ISA         = qw(Exporter);
+    %EXPORT_TAGS = ();
+}
+
+# Several routines to work with arrays whose elements are unique
+# (here called sets)
+
+=head1 NAME
+
+Devscripts::Set - Functions for handling sets.
+
+=head1 SYNOPSIS
+
+use Devscripts::Set;
+
+@set=ListToSet(@list);
+
+@setdiff=SetMinus(\@set1,\@set2);
+
+@setinter=SetInter(\@set1,\@set2);
+
+@setunion=SetUnion(\@set1,\@set2);
+
+=head1 DESCRIPTION
+
+ListToSet: Make a set (array with duplicates removed) from a list of
+items given by an array.
+
+SetMinus, SetInter, SetUnion: Compute the set theoretic difference,
+intersection, union of two sets given as arrays.
+
+=cut
+
+# Transforms a list to a set, removing duplicates
+# input:  list
+# output: set
+
+sub ListToSet (@) {
+    my %items;
+
+    grep $items{$_}++, @_;
+
+    return keys %items;
+}
+
+# Compute the set-theoretic difference of two sets.
+# input: ref to Set 1, ref to Set 2
+# output: set
+
+sub SetMinus ($$) {
+    my ($set1, $set2) = @_;
+    my %items;
+
+    grep $items{$_}++, @$set1;
+    grep $items{$_}--, @$set2;
+
+    return grep $items{$_} > 0, keys %items;
+}
+
+# Compute the set-theoretic intersection of two sets.
+# input: ref to Set 1, ref to Set 2
+# output: set
+
+sub SetInter ($$) {
+    my ($set1, $set2) = @_;
+    my %items;
+
+    grep $items{$_}++, @$set1;
+    grep $items{$_}++, @$set2;
+
+    return grep $items{$_} == 2, keys %items;
+}
+
+#Compute the set-theoretic union of two sets.
+#input: ref to Set 1, ref to Set 2
+#output: set
+
+sub SetUnion ($$) {
+    my ($set1, $set2) = @_;
+    my %items;
+
+    grep $items{$_}++, @$set1;
+    grep $items{$_}++, @$set2;
+
+    return grep $items{$_} > 0, keys %items;
+}
+
+1;
+
+=head1 AUTHOR
+
+Bill Allombert <ballombe@debian.org>
+
+=head1 COPYING
+
+Copyright 2001 Bill Allombert <ballombe@debian.org>
+Modifications Copyright 2002 Julian Gilbey <jdg@debian.org>
+dpkg-depcheck is free software, covered by the GNU General Public License, and
+you are welcome to change it and/or distribute copies of it under
+certain conditions.  There is absolutely no warranty for dpkg-depcheck.
+
+=cut