clue/graph — pull request #72 by clue (2013-09-11 10:13:38+00:00)
Title: A null graph is not a valid tree

* [x] Make sure a null Graph (a Graph with no Vertices and thus no Edges) is not a valid tree (because it's not connected) (BC break!)
* [x] Documentation
* [ ] Changelog

See also http://mathoverflow.net/questions/120536/is-the-empty-graph-a-tree