The execution of a Petri-net is controlled by the position and movement of markers (called tokens) in the Petri-net. Tokens, indicated by black dots, reside in the circles representing the places of a net. A Petri-net with tokens is called a marked Petri-net. Tokens are moved by the firing of the transitions of the net. A transition must be enabled in order to fire. (A transition is enabled when all of its input places have a token in them). The transition fires by removing the enabling tokens from their input places and generating new tokens which are deposited in the output places of the transition.
As an example, in the marked Petri-net of Figure 2, the transition t2 is enabled since it has a token in its input place (p1). If t2 fires, the marked Petri-net of Figure 3 results. The firing of transition t2 removes the enabling token from place p1 and puts tokens in p2 and p3, the two outputs of t2.
