Unlink given node and hyperedge.

        @type  node: node
        @param node: Node.

        @type  hyperedge: hyperedge
        @param hyperedge: Hyperedge.

def unlink(self, node, hyperedge):
        """
        Unlink given node and hyperedge.

        @type  node: node
        @param node: Node.

        @type  hyperedge: hyperedge
        @param hyperedge: Hyperedge.
        """
        self.node_links[node].remove(hyperedge)
        self.edge_links[hyperedge].remove(node)
        self.graph.del_edge(((node,'n'), (hyperedge,'h')))