@Contract(pure=true)
  public static int @NotNull [] toIntArray(@NotNull Collection<Integer> list) {
    int[] ret = newIntArray(list.size());
    int i = 0;
    for (Integer e : list) {
      ret[i++] = e;
    }
    return ret;
  }
@Contract(pure=true)
  public static int @NotNull [] toIntArray(@NotNull Collection<Integer> list)