threaded balanced tree

Hello, Does anyone have a purely functional haskell implementation of a threaded, balanced tree? I modified Okasaki's SML red-black tree implementation, but my modifications mutate. I would be very interested in seeing an approach that doesn't use mutability. Thanks Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein@eecs.harvard.edu cell: 617-233-0246