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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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