Link given node and hyperedge.

        @type  node: node
        @param node: Node.

        @type  hyperedge: node
        @param hyperedge: Hyperedge.

def link(self, node, hyperedge):
        """
        Link given node and hyperedge.

        @type  node: node
        @param node: Node.

        @type  hyperedge: node
        @param hyperedge: Hyperedge.
        """
        if (hyperedge not in self.node_links[node]):
            self.edge_links[hyperedge].append(node)
            self.node_links[node].append(hyperedge)
            self.graph.add_edge(((node,'n'), (hyperedge,'h')))
        else:
            raise AdditionError("Link (%s, %s) already in graph" % (node, hyperedge))