The NetHack General Public License is a copyleft software license primarily used by the roguelike game NetHack. It is certified as an open source license by the Open Source Initiative. The license was written in 1989 by Mike Stephenson, who patterned it after the GNU bison license (which was written by Richard Stallman in 1988). Like the Bison license, and Stallman's later GNU General Public License, the NetHack license was written to allow the free sharing and modification of the source code under its protection. At the same time, the license explicitly states that the source code is not covered by any warranty, thus protecting the original authors from litigation.