Mariko Sakashita Kazuhisa Makino Hiroshi Nagamochi Satoru Fujishige Minimum Transversals in Posi-modular Systems. 576-587 2006 conf/esa/2006 ESA http://dx.doi.org/10.1007/11841036_52 db/conf/esa/esa2006.html#SakashitaMNF06