Функция сложения бинарных деревьев через fix на Haskell

data BinTree a = Leaf a | Node ( BinTree a) ( BinTree a)
    deriving (Show, Eq, Ord)
 
instance Num a => Num (BinTree a) where
    (+) (Leaf _) x = x
    (+) (Node l r) x = Node (l + x) r
    (*) (Leaf a) (Leaf b) = Leaf (a*b)
    (*) (Node l r) (Node l1 r1) = Node(l*l1) (r*r1)
    (*) _ _ = error "oshibka"

repeat :: a -> [a]
repeat = fix . (:) 
 
map :: (a -> b) -> [a] -> [b]
map f = fix $ \m xs -> case xs of
[]   -> []
y:ys -> f y : m ys

import  Data.Function (fix)
 
data BinTree a = Leaf a | Node ( BinTree a) ( BinTree a)
    deriving (Show, Eq, Ord)
 
treeAdd :: Num a => BinTree a -> BinTree a -> BinTree a
treeAdd = fix $ \m tl tr -> case (tl,tr) of
            ((Leaf _),x) -> x
            ((Node l r),x) -> Node (m l x) r
 
testTree :: BinTree Int
testTree= Node
            (Node 
                (Node 
                    (Leaf 6) 
                    (Node
                        (Node (Leaf 5) (Leaf 2))
                        (Leaf 9))) 
                (Node (Leaf 7) (Leaf 8)))
            (Node 
                (Node 
                    (Leaf 8) 
                    (Node (Leaf 4) (Leaf 9))) 
                (Node (Leaf 3) (Leaf 1)))
 
main :: IO ()
main = print $ treeAdd testTree testTree