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A group of MIT students built a hack kit that turned Pac-Man into a harder, better game, Midway bought it and shipped it as a sequel, and Namco in Japan found out afterward. It out-sold the original in America.
The four mazes and the random ghost movement made it the version serious players preferred, and it's the one still running in bars, often in the same cabinet as Galaga.
Sources: Wikipedia — Ms. Pac-Man