This is a port of Wolf4SDL, which is derived from the original id
software source release. The port runs on top of the SDL plugin
runtime and is loaded as an overlay.
Licensing of the game code is not an issue, as discussed below
(essentially, the Debian project treats Wolf4SDL as GPLv2, with an
email from John Carmack backing it up):
Included is a copy of MAME's Yamaha OPL sound chip emulator
(fmopl_gpl.c). This file was not part of the original Wolf4SDL source
(which includes a non-GPL'd version), but was rather rebased from from
a later MAME source which had been relicensed to GPLv2.