Rajeev Alur Radu Grosu Insup Lee Oleg Sokolsky Compositional Refinement for Hierarchical Hybrid Systems. 33-48 2001 conf/hybrid/2001 HSCC http://link.springer.de/link/service/series/0558/bibs/2034/20340033.htm db/conf/hybrid/hscc2001.html#AlurGLS01