A Petri net example. A Petri net, whose places p 1 and p 2 form a p-invariant, representing for example a logical negation. p 1 can be a condition A and p 2 a condition ¬A, or vice versa, indicating two states of the system, for example an activated or deactivated protein. The place p 1 carries one token to indicate in which state the system is. Transitions t 3 and t 4 are connected to this p-invariant by read arcs in order to hold the token on those places, which form the p-invariant.