Thomas R. Shiple Ramin Hojati Alberto L. Sangiovanni-Vincentelli Robert K. Brayton Heuristic Minimization of BDDs Using Don't Cares. 225-231 1994 DAC http://doi.acm.org/10.1145/196244.196360 db/conf/dac/dac94.html#ShipleHSB94