balance :: (Tree a) -> Tree a
balance (Node l a r)
	| skew (Node l a r) == 2
		| skew l == ~1 = leftRightRot (Node l a r)
		| otherwise = rightRot (Node l a r)
	| skew (Node l a r) == ~2
		| skew r == 1 = rightLeftRot (Node l a r)
		| otherwise = leftRot (Node l a r)
	| otherwise = Node l a r (avlInsert x ab))