fun focus (x :: xs, ys) = SOME (x, xs, ys)
  | focus _ = NONE

fun first xs = focus (xs, nil)
fun next (x, xs, ys) = focus (xs, x :: ys)

fun focused (x, _, _) = x
fun restore (x, xs, ys) = (rev ys, x :: xs)

fun loop (_, NONE) = NONE
  | loop (x, SOME xs) =
    if x = focused xs then
      SOME xs
    else
      loop (x, next xs)

fun splitAtFirst (x, xs) =
  case loop (x, first xs) of
      NONE => (xs, nil)
    | SOME xs => restore xs

fun loop (_, xs, NONE) = xs
  | loop (x, xs, SOME ys) =
    if x = focused ys then
      loop (x, SOME ys, next ys)
    else
      loop (x, xs, next ys)

fun splitAtLast (x, xs) =
  case loop (x, NONE, first xs) of
      NONE => (xs, nil)
    | SOME xs => restore xs