28.7. Background
A plethora of process modeling and data flow diagramming languages have been around since forever. Each year, UML activity diagrams become more popular as a common standard, though there is still significant variation.The semantics of activity diagrams are loosely based on Petri nets , an important computational theory in computer science. The metaphoror actualizationof Petri nets is that there are tokens flowing through the activity graph. For example, when a token arrives at an action node, it executes. When all the required input tokens arrive at a join, an output token is created.