Cityville Energy Refill X Windows itself was free software, but mainly as a way to level the playing field between competing business interests, not out of some desire to end the dominance of proprietary software. Yet another example, predating the GNU project by a few years, was TeX, Donald Knuth‘s free, publishing-quality typesetting system. Cityville Energy Refill released it under a license that allowed anyone to modify and distribute the code, but not to call the result "TeX" unless it passed a very strict set of compatibility tests (this is an example of the "tradcmark-protecting" class of free licenses, discussed more in Chapter 9, Licenses, Copyrights, and Patents). Knuth wasn’t taking a stand one way or the other on the question of free versus proprietary software, he just needed a better typesetting system in order to complete his real goal a book on computer programming and Cityville Energy Refill saw no reason not to release his system to the world when done.