* a request itself.
   *
   * @since 31.0
   */
  public static final String X_DEVICE_REQUESTED_WITH = "X-Device-Requested-With";
  /** The HTTP {@code X-Do-Not-Track} header field name. */
  public static final String X_DO_NOT_TRACK = "X-Do-Not-Track";
  /** The HTTP {@code X-Forwarded-For} header field name (superseded by {@code Forwarded}). */
  public static final String X_FORWARDED_FOR = "X-Forwarded-For";

    @CheckForNull private Iterator<? extends T> toRemove;

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*
     * We track the "meta iterators," the iterators-of-iterators, below.  Usually, topMetaIterator
     * is the only one in use, but if we encounter nested concatenations, we start a deque of