Matthew, Yuki_tree.take_while should not stop at an empty node: just skip over it

To do: setup make upgrade

By the way, there is a finger tree implementation in coq.
http://coq.inria.fr/distrib/v8.2/contribs/FingerTree.DependentFingerTreeModule.html
http://mattam.org/research/russell/fingertrees.en.html