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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 has_successor(self, u, v):

 """
 Check if vertex u has successor v
 """

        if u not in self.vertices:
            raise GraphInsertError("Vertex %s doesn't exist." % (u,))
        return (u in self.succ and v in self.succ[u])