}

    /**
     * Returns {@code this} up-casted to the specific {@link Segment} implementation type {@code S}.
     *
     * <p>This method exists so that the {@link Segment} code can be generic in terms of {@code S},
     * the type of the concrete implementation.
     */
    abstract S self();

    /** Drains the reference queues used by this segment, if any. */
    @GuardedBy("this")

  /**
   * Determines whether a character is a breaking whitespace (that is, a whitespace which can be
   * interpreted as a break between words for formatting purposes). See {@link #whitespace()} for a
   * discussion of that term.
   *
   * @since 19.0 (since 2.0 as constant {@code BREAKING_WHITESPACE})
   */
  public static CharMatcher breakingWhitespace() {
    return BreakingWhitespace.INSTANCE;
  }

  /**

  /**
   * Determines whether a character is a breaking whitespace (that is, a whitespace which can be
   * interpreted as a break between words for formatting purposes). See {@link #whitespace()} for a
   * discussion of that term.
   *
   * @since 19.0 (since 2.0 as constant {@code BREAKING_WHITESPACE})
   */
  public static CharMatcher breakingWhitespace() {
    return BreakingWhitespace.INSTANCE;
  }

  /**