The Double Metaphone algorithm is copied from the Perl library that holds the following copyright notice. To the best of our knowledge the Perl license is compatible to the SWI-Prolog license schema and therefore including this module poses no additional license conditions.
Copyright 2000, Maurice Aubrey <email@example.com>. All rights reserved.
This code is based heavily on the C++ implementation by Lawrence Philips and incorporates several bug fixes courtesy of Kevin Atkinson <firstname.lastname@example.org>.
This module is free software; you may redistribute it and/or modify it under the same terms as Perl itself.