PushPush is NP-hard in 3D - Joseph O'Rourke; Beenish Chaudry; Sorina Chircu; Elizabeth F. Churchill; Sasha Fedorova; Judy A. Franklin; Biliana Kaneva; Halley Miller; Anton Okmianski; Irena Pashchenko; Ileana Streinu; Geetika Tewari; et al. (CoRR, 1999)