I had no time to read the source yet. I was wondering, did you use any of the work from Chris Okasaki (Purely Functional Data Structures) or it is irrelevant to your project?
However, many of the data structures in that book do not translate great outside of the world of Haskell and ML. This library is mostly based on the work of Phil Bagwell, Rich Hickie, Tiark Rompf and others. Phil worked over the last decade on building immutable and/or concurrent data structures that are "pragmatic", ie. cache efficient, and supporting mutable facades. I recommend reading his work on Hash-Array Mapped Tries and Radix Balanced Trees.