generic finger-tree structure (http://staff.city.ac.uk/~ross/papers/FingerTree.html)
#16Can we have a function `:: FingerTree v a -> v`?
I'd like to use this library, but also fetch the v value for the entire sequence (or prefixes of the sequence obtained from split). Would it be okay to add a function to access this? I don't see why it would be unsafe, and it's definitely not hard to implement!
