Fig. 3
From: A methodological approach for using high-level Petri Nets to model the immune system response
Example of the use of a stochastic transition to model entities aging. The guard on the stochastic transition randomly selects only tokens that represent TH cells whose life is greater than 0 (th:3 > 0), and gives back to the starting place a new token that is identical to the previously selected one, except for the fact that LIFE has been decremented by 1 (1’(th:1,th:2,th:3–1,th:4))