Moreover, every arrow is labeled with a terminal identifier. Dashed arrows are called input arrows , representing the occurrence of an input signal on the associated terminal. Similarly, solid arrows are called output arrows , representing the occurrence of an output signal on the associated terminal. The bold filled versions on the lower line are used for the initial state . For every node, the arrows leaving that node have distinct labels.
All nodes have identical sets of outgoing arrow labels.
Every node is reachable from the initial node via a path of zero or more arrows. A transient node has at least one outgoing output arrow. A demanding node has at least one outgoing input arrow. An input arrow to a top node comes from a top node. An output arrow to a bottom node comes from a bottom node.