Note that these are possibilities, not plans nor promises.
1. separate external and internal attributes?
2. biconn: add next_root
3. DAG SSSP
4. flow: Ford-Fulkerson/Edmonds-Karp/preflow-push?
-- Classics --
Undirected graphs
- connectivity
	- given two vertices are they on a cycle
	- find_all_paths($u, $v)?
	- find_all_cycles()?
- Euler tour
- bipartite matching
- 2-colorability
Digraphs
- disallow_selfloops => 1?
- odd-length cycle
- shortest paths (bfs)
- biconnected components
- girth (shortest cycle)?
