instance Num a => Num (Tree a) where
    (+) (Leaf _) x = x
    (+) (Branch l r) x = Branch (l+x) r
    (*) (Leaf a) (Leaf b) = Leaf (a + b)
    (*) (Branch l1 r1) (Branch l2 r2) = Branch (l1*l2) (r1*r2)
    (*) _ _ = error "Undefined"