-- linear implementation of reverse using an accumulating argument
rev :: [Int] -> [Int] -> [Int]
rev xs = iter xs []
 where
  iter []     rxs = rxs
  iter (x:xs) rxs = iter xs (x:rxs)

-- insert at arbitrary position
insert :: Int -> [Int] -> [[Int]]
insert x []     = [[x]]
insert x (y:ys) = (x:y:ys) : map (y:) (insert x ys)

-- compute all permutations
perm []     = [[]]
perm (x:xs) = concat (map (insert x) (perm xs))