An editing utility, often called an editor, is a program that allows a user to create or modify portions of the games using or based on the Doom engine. Doom is recognized as one of the earliest games to explicitly support the modification of the game for the extension of gameplay, through the use of arbitrary WAD files (PWADs). With all the technical information now known about the Doom games, Heretic, and Hexen, it is possible to modify nearly anything and everything within the Doom environment. Some of the utilities that enable this are listed below.