app app map f nil nil app app map f app app cons x xs app app cons app f x app app map f xs app app append xs nil xs app app append nil ys ys app app append app app cons x xs ys app app cons x app app append xs ys app app zip nil yss yss app app zip xss nil xss app app zip app app cons xs xss app app cons ys yss app app cons app app append xs ys app app zip xss yss app app combine xs nil xs app app combine xs app app cons ys yss app app combine app app zip xs ys yss app levels app app node x xs app app cons app app cons x nil app app combine nil app app map levels xs app 2 map 0 nil 0 cons 0 append 0 zip 0 combine 0 levels 0 node 0 FULL ./TRS/higher-order/Bird/TreeLevels.trs