code: def remove_observer(self, signal, observer):
          
  
          self._is_allowed_event(signal)
          self._remove_observer(signal, observer)
docs: Remove an observer from the object.
  
          Raise an eception if the signal is not allowed.
  
          Parameters
          ----------
          signal : str
              a valid signal.
          observer : @func
              an obervation function to be removed.