Alessandro Giua Daniele Corona Carla Seatzu State Estimation of lambda-free Labeled Petri Nets with Contact-Free Nondeterministic Transitions*. 85-108 2005 15 Discrete Event Dynamic Systems 1 http://dx.doi.org/10.1007/s10626-005-5239-4 db/journals/deds/deds15.html#GiuaCS05