Hello folks, I've put and AVL tree based clone of Data.Set here.. http://homepages.nildram.co.uk/~ahey/HLibs/AvlSet/AvlSet.html This should make it easy to do comparitive test/measurements between the two and maybe experiment with any wishlist extensions you want for the revised API (which is WIP I believe), using the underlying AVL implementation. I'll do the same thing for Data.Map soon, though it'll probably have to wait until next weekend now. This module is completely untested but the underlying AVL library has been pretty thouroughly tested. So it should work OK. If you use it and find any problems please let me know. Regards -- Adrian Hey