Class Cooperate

Instruction --+    
              |    
      CallLater --+
                  |
                 Cooperate

Requests that processing be paused so other tasks can resume

Yield this object when the current chain would block or periodically
during an intensive processing task.   The flow mechanism uses these
objects to signal that the current processing chain should be paused
and resumed later.  This allows other delayed operations to be
processed, etc.  Usage is quite simple:

       // within some generator wrapped by a Controller
       yield Cooperate(1)  # yield for a second or more