The CWI license is the license applied to early releases of Hack, as detailed here. CWI, the Centrum voor Wiskunde en Informatica, is the new name for the Stichting Mathematisch Centrum. It is a BSD-type license, and one-way (inclusion) compatible with the GNU GFDL. For the purposes of Wikihack, that is perfectly adequate. NetHack 1.3d, 1.4f, 2.2a and 2.3e contain no explicit license statement, but include the line: /* Copyright (c) Stichting Mathematisch Centrum, Amsterdam, 1985. */ which probably means the CWI license applies.