Roland Glück Bernhard Möller Michel Sintzoff A Semiring Approach to Equivalences, Bisimulations and Control. 134-149 2009 RelMiCS http://dx.doi.org/10.1007/978-3-642-04639-1_10 conf/RelMiCS/2009 db/conf/RelMiCS/RelMiCS2009.html#GluckMS09