Computation sequence sets

A class of automata based upon generalized Petri nets is introduced and defined. The language which a Petri net generates during an execution is called a computation sequence set (CSS). The class of CSS languages is shown to be closed under union, intersection, concatenation, and concurrency. All regular languages and all bounded context-free languages are CSS, while all CSS are context-sensitive. Not all CSS languages are context-free, nor are all context-free languages CSS. Decidability problems for CSS hinge on the emptiness problem for CSS. This problem is equivalent to the reachability problem for vector addition systems, and is open