Fig 10.6. A simple Petri net example.
Initially input places p1 and p2 contain three and two tokens, respectively. By firing transition t one token will be removed from p1 and two tokens from p2. Transition t will consume tokens and place two of them into place p3. Transition t may fire since its preplaces p1 and p2 have sufficient number of tokens. After one firing step the marking of the net is changed: p1 has only two tokens left, p2 has no tokens, and p3 has two tokens. Transition t cannot fire anymore.