Fork : A fork spawns（产生） multiple concurrent(并发) paths of execution.
<process-definition name="the fork-in-loop process">
<swimlane name="initiator"/>
<start-state name="start" swimlane="initiator">
<transition to="aFork"/>
</start-state>
<fork name="aFork">
<transition to="aState"/>
</fork>
<state name="aState" swimlane="initiator">
<transition to="aJoin"/>
</state>
<join name="aJoin">
<transition to="aFork"/>
</join>
<end-state name="end"/>
</process-definition>