Eliminate constraints which mention objects not in 'objects'.
            In graph-theory terms, this is finding subgraph induced by
            ordered vertices.

def __eliminate_unused_constraits (self, objects):
        """ Eliminate constraints which mention objects not in 'objects'.
            In graph-theory terms, this is finding subgraph induced by
            ordered vertices.
        """
        result = []
        for c in self.constraints_:
            if c [0] in objects and c [1] in objects:
                result.append (c)

        return result