Extracting siphons containing a specified set of places in a Petri net

Given a Petri net PN=(P, T, E), a siphon is a set S of places such that the set of input transitions to S is included in the set of output transitions from S. Concerning minimal siphons containing a specified set of places, the paper shows several NP-completeness results of and a branch-and-bound method of extracting such one minimal siphon, as well as a method of enumerating all such ones containing a given place.