A activity which completes when quorum of the child activities are completed
successfully (or there are too many child activities failed to achieve
quorum).
Represents a piece of state which can be accessed in a multi threaded way
rather like
java.util.concurrent.atomic.AtomicReference but which
also supports the use of listeners to be notified when the state changes.
Represents an arbitrary step on a workflow object which can be
used by enumerations to define the workflow transitions and what happens at each
transition.