fun quicksort nil = nil
|   quicksort (pivot :: rest) =
      let
        fun split(nil) = (nil,nil)
        |   split(x :: xs) =
              let
                val (below, above) = split(xs)
              in
                if x < pivot then (x :: below, above) 
                else (below, x :: above)
              end;
        val (below, above) = split(rest)
      in
        quicksort below @ [pivot] @ quicksort above
      end;