privat/nit — pull request #524 by Morriar (2014-06-25 19:27:10+00:00)
Title: ordered_tree: add `is_empty` and `to_a` method

Add some missing methods to OrderedTree.

Well... at this point I highly consider to make OrderedTree to implement Collection but there is some tricky services to implement efficiently in the OrderedTree.

Signed-off-by: Alexandre Terrasa