Log Message:
Import fingertree-0.1.4.2 from wip

A general sequence representation with arbitrary annotations, with
example implementations of various collection types, as described in
section 4 of

    Ralf Hinze and Ross Paterson, "Finger trees: a simple
    general-purpose data structure", Journal of Functional Programming
    16:2 (2006) pp
    197-217. http://www.soi.city.ac.uk/~ross/papers/FingerTree.html

For a tuned sequence type, see Data.Sequence in the containers
package, which is a specialization of this structure.