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Near infinitely replayable games that usually feature, random generation, perma-death, and brutal difficulty. 

Shoot 'Em Up or shmup games are a subset of shooter games where the player continually moves forward while waves of enemies enter the screen. Both shooting and dodging are critical to advancing. See also Bullet Hell.

Games where the primary goal is keeping the player character alive under harsh circumstances such as dwindling resources or a hostile environment.