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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 get_vertex_faces(self):
 """ List mapping each vertex index to a list of face indices that use it. """

        if self._vertex_faces is None:
            self._vertex_faces = [[] for i in xrange(len(self.get_vertices()))]
            for i in xrange(self._faces.shape[0]):
                face = self._faces[i]
                for ind in face:
                    self._vertex_faces[ind].append(i)
        return self._vertex_faces