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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 referenced_vertices(self):
 """ Which vertices are referenced by an entity. Returns referenced_vertices: (n,) int, indexes of self.vertices """

        # no entities no reference
        if len(self.entities) == 0:
            return np.array([], dtype=np.int64)
        referenced = np.concatenate([e.points for e in self.entities])
        referenced = np.unique(referenced.astype(np.int64))

        return referenced