fun zipOpt (inlx, inly) =
  let
    fun zipOptI (_, [], ans) = rev ans
      | zipOptI (NONE :: ls, xl, ans) = zipOptI (ls, xl, ans)
      | zipOptI ([], yl, ans) = zipOptI (inlx, yl, ans)
      | zipOptI (SOME x :: xl, y :: yl, ans) = zipOptI (xl, yl, (x, y) :: ans)
  in
    zipOptI (inlx, inly, [])
  end