Returns the relation operation from the last node visited on the
        path, or `None`, if no node has been visited yet.

def relation_operation(self):
        """
        Returns the relation operation from the last node visited on the
        path, or `None`, if no node has been visited yet.
        """
        if len(self.nodes) > 0:
            rel_op = self.nodes[-1].relation_operation
        else:
            rel_op = None
        return rel_op