namespace ListInternal =

  // -- Flatmap ----------------------------------------------------------------

  function flat_map(f : 'a => list('b), xs : list('a)) : list('b) =
    switch(xs)
      []      => []
      x :: xs => f(x) ++ flat_map(f, xs)

  // -- From..to ---------------------------------------------------------------

  function from_to(a : int, b : int) : list(int) = from_to_(a, b, [])

  private function from_to_(a, b, acc) =
    if (a > b) acc else from_to_(a, b - 1, b :: acc)