code: def unblock(self, callback=None):
          
          if callback not in self._blocked or self._blocked[callback] == 0:
              raise RuntimeError("Cannot unblock %s for callback %s; emitter "
                                 "was not previously blocked." % 
                                 (self, callback))
          b = self._blocked[callback] - 1
          if b == 0 and callback is not None:
              del self._blocked[callback]
          else:
              self._blocked[callback] = b
docs: Unblock this emitter. See :func:`event.EventEmitter.block`.
          
          Note: Use of ``unblock(None)`` only reverses the effect of 
          ``block(None)``; it does not unblock callbacks that were explicitly 
          blocked using ``block(callback)``.