A Petri net is a class of directed graphs having two types of nodes, place
nodes and transition nodes, where branches connect nodes of different types, and
where place-nodes have (“ contain ”) associated objects called tokens. For any
transition-node T, branches from a place-node to the transition-node, or from
the transition-node to a place-node, define the input place-nodes (or preset) and
output place-nodes (or postset), respectively, of the given transition-node.