Simulationism maintains a self-contained universe operating independent of player will; events unfold according to internal rules. Combat may be broken down into discrete, semi-randomised steps for modeling attack skill, weapon weight, defense checks, armor, body parts and damage potential. Some simulationist RPGs explore different aspects of their source material, and may have no concern for realism; Toon, for example, emulates cartoon hijinks. Role-playing game systems such as GURPS and Fudge use a somewhat-realistic core system which can be modified with sourcebooks or special rules