I think that your compare function has the result inverted, you could
try:

between a b x
 | x < a     = GT
 | x > b     = LT
 | otherwise = EQ

I think that the tree is correct.

On Fri, 2010-09-17 at 08:41 +0100, Stephen Tetley wrote:
> Hello Alexander
>
> I suspect mydelete' isn't working because it needs a "sorted" tree -
> either the initial tree you are supplying isn't sorted or it becomes
> unsorted after one delete step.
>
> I don't know the AVL library at all (I only installed it now to look
> at your problem) so I can't say from looking at the constructors
> whether the problem tree is malformed, the problem tree being:
>
> > P (Z E 1 E) 3 E
>
> Potentially very few people use the Avl module which is why you
> haven't had a reply yet. You could try posting to the Cafe instead if
> you don't get a better answer.
>
> Best wishes
>
> Stephen

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