code: def callback(self, result):
          
          self._setResult(result)
          self._isFailure = False
  
          for d in self._deferreds:
              d.callback(result)
docs: Callbacks the deferreds previously produced by this object.
  
          @param result: The object which will be passed to the
          C{callback} method of all C{Deferred}s previously produced by
          this object's C{tee} method.
          @raise AlreadyCalledError: If L{callback} or L{errback} has
          already been called on this object.