English: a | A Boolean network. Each of the entities a, b and c in the network can be in state 0 or 1. State transitions obey the regulation functions shown on the right, which describe the rules of the model. For example, if a is in state 1 and c is in state 0, at the next time step the state of b will be 0. Thin arrows indicate the regulators of each node. Time steps are represented by thick arrows. The global state of the model is the combination of the three entity states. The system cycles through the six global states. A sequence of consecutive global states is called a trajectory. b | A Petri net. The net contains 'places' (light blue circles) that are the model's entities, and 'transitions' (rectangles) that constitute the regulation functions and define the model's dynamics. Arcs connect input places to transitions, and transitions to their output places. Places that receive discrete values are called tokens (dark blue dots). A transition that is activated, or 'fired', reduces the tokens in its input places and increases the number of tokens in each of its output places. At any time step, every transition that has enough tokens in its input places may be fired. In the example, every transition consumes one token from every input place, and produces one token at every output place. Labels at thick arrows indicate which transition fired. Transitions t1 and t3 can be fired in alternation indefinitely, whereas no other transition can be fired after t2 has fired.