invalidate :: RotationState a -> RotationState a
invalidate (Reversing ok f f' r r') = Reversing (ok - 1) f f' r r'
invalidate :: RotationState a -> RotationState a
invalidate (Reversing ok f f' r r') = Reversing (ok - 1) f f' r r'
invalidate (Reversing ok f f' r r') = Reversing (ok - 1) f f' r r'