The F# Journal just published an article:
"This is the second article in a series about balanced search trees. This article takes a look at a purely functional implementation of the AA tree data structure as described by Arne Andersson in the paper "Balanced Search Trees Made Simple". In particular, the brevity of our implementation is compared to that of the full red-black tree detailed in the previous article. In this case we test not only the externally-visible effects of the collection but also its invariants..."