AVL library update

Hello, I've put a new version 2.3 of Data.Tree.AVL here.. http://homepages.nildram.co.uk/~ahey/HLibs/Data.Tree.AVL/ Changes in this version include.. * Added genPush' and genPushMaybe' functions which provide strictness control which was missing from earlier versions. * Added symmetric difference to the set functions * I decided to use the prefix "assert" for partial function names. So there has been a bit of renaming (old names are still OK but deprecated, and I might have missed a few :-) Regards -- Adrian Hey