Fix bug introduced while documenting PostOrderIterator

    :25            dwins | looks like i rearranged some code
                           while i was adding doc comments on                                                                                                                                  
                           tuesday and screwed postorderiterator
                           up badly
    :26 JohnathanGarrett | hmm I suppose that would explain why
                           I wasn't seeing the same thing as
                           you, I haven't updated to the latest
                           master yet
    :27            dwins | yes it would
    :28            dwins | basically only the first item at a
                           given depth would actually be
                           returned from a visitor
    :28            dwins | so for example walk-graph would only
                           encounter 94 features on the sample
                           repo you passed along earlier