has

partreeMap :: (a -> b) -> BinaryTree a -> BinaryTree b
partreeMap f Leaf = Leaf
partreeMap f (Branch v l r) = let v' = f v
                         l' = partreeMap f l
                         r' = partreeMap f r
                           in
                           (v' `par` l' `par` r') `pseq` (Branch v' l' r')