parallel, simulation A directed, bipartite graph in

which nodes are either "places" (represented by circles) or

"transitions" (represented by rectangles), invented by Carl

Adam Petri. A Petri net is marked by placing "tokens" on

places. When all the places with arcs to a transition (its

input places) have a token, the transition "fires", removing a

token from each input place and adding a token to each place

pointed to by the transition (its output places).