Check if attributes given to the constructor can be used to
        instanciate a valid node.

def _check_attributes(self, attributes, extra=None):
        """Check if attributes given to the constructor can be used to
        instanciate a valid node."""
        extra = extra or ()
        unknown_keys = set(attributes) - set(self._possible_attributes) - set(extra)
        if unknown_keys:
            logger.warning('%s got unknown attributes: %s' %
                            (self.__class__.__name__, unknown_keys))