*
   * <p>In order for "::" compression to work, the input should contain negative sentinel values in
   * place of the elided zeroes.
   *
   * @param hextets {@code int[]} array of eight 16-bit hextets, or -1s
   */
  private static String hextetsToIPv6String(int[] hextets) {
    // While scanning the array, handle these state transitions:
    //   start->num => "num"     start->gap => "::"