instance Num a => Num (BinTree a) where
    (+) (Leaf a) x = x
    (+) (Branch l r) x = Branch (l + x) r  -- (+) l x

    (*) (Leaf a) (Leaf b) = Leaf (a * b)
    (*) (Branch l r) (Branch l1 r1) = Branch (l * l1) (r * r1)
    (*) _ _ = error "neponyal"