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We present a demonstration of Tanagra, an intelligent level design assistant for 2D platformers. Tanagra integrates reactive planning and constraint programming to automatically generate levels and respond to designer changes in realtime. A reactive planning language, ABL, allows us to easily express hierarchical patterns of level geometry, from simple patterns such as a jump over a gap, to more complex patterns such as staircases and pits. A model of player movement and constraints within and between geometry components ensure that the levels created by our system are always playable.