code: public static ImmutableIntArray of(int first, int... rest) {
      checkArgument(
          rest.length <= Integer.MAX_VALUE - 1, "the total number of elements must fit in an int");
      int[] array = new int[rest.length + 1];
      array[0] = first;
      System.arraycopy(rest, 0, array, 1, rest.length);
      return new ImmutableIntArray(array);
    }
docs: okay since we have to copy the just-created array anyway.