Slump is relesed under the GPL with two optional requests added to the license:

Copyright 2000 David Chess; Copyright 2005 Sam Trenholme

Slump 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, or (at your option) any later
version.

Slump 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 Slump; see the file GPL.  If not, write to the Free
Software Foundation, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA.

Additionally, while not required for redistribution of this program, 
the following requests are made when making a derived version of 
this program:

* Slump's code is partly derived from the Doom map generator 
  called SLIGE, by David Chess.  Please inform David Chess of 
  any derived version that you make.  His email address is at 
  the domain "theogeny.com" with the name "chess" placed before 
  the at symbol.

* Please do not call any derivative of this program SLIGE.