Token:  Threads directly in a process instance (i.e., not threads spawned
    implicitly).
    The Root Token and new threads made by...  [TODO:  add calls]

GRAPH XML LANGUAGE:  JPDL
    A single graph is one process definition

SIGNAL = resume execution

Can be persisted only in an "overall wait state".  I.e., all tokens are
in wait states.

EVENTS:  Leaving a node; entering a node; taking a transition.

ACTIONS:  Java code executed upon EVENTs.
    These are implemented by developer and do not modify the graph.