def node_graph(self):
g = {}<EOL>for i, j in self.edge_db:<EOL><INDENT>s = g.setdefault(i, set())<EOL>s.add(j)<EOL>s = g.setdefault(j, set())<EOL>s.add(i)<EOL><DEDENT>return g<EOL>
Return a graph of node_id's pointing to node_id's.

        The arcs of the graph correspond to the edges in the triangulation.

        {node_id: set([node_id, ...]), ...}