public static CharMatcher invisible() {
    return Invisible.INSTANCE;
  }

  /**
   * Determines whether a character is single-width (not double-width). When in doubt, this matcher
   * errs on the side of returning {@code false} (that is, it tends to assume a character is
   * double-width).
   *
   * <p><b>Note:</b> as the reference file evolves, we will modify this matcher to keep it up to
   * date.
   *

   * href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Viewport-Width">{@code
   * Viewport-Width}</a> header field name.
   *
   * @since 31.0
   */
  public static final String VIEWPORT_WIDTH = "Viewport-Width";

  /**
   * The HTTP <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Width">{@code
   * Width}</a> header field name.
   *
   * @since 31.0
   */

  public static CharMatcher invisible() {
    return Invisible.INSTANCE;
  }

  /**
   * Determines whether a character is single-width (not double-width). When in doubt, this matcher
   * errs on the side of returning {@code false} (that is, it tends to assume a character is
   * double-width).
   *
   * <p><b>Note:</b> as the reference file evolves, we will modify this matcher to keep it up to
   * date.
   *

   *       spend K saved permits.
   * </ol>
   *
   * <p>In summary, the time it takes to move to the left (spend K permits), is equal to the area of
   * the function of width == K.
   *
   * <p>Assuming we have saturated demand, the time to go from maxPermits to thresholdPermits is
   * equal to warmupPeriod. And the time to go from thresholdPermits to 0 is warmupPeriod/2. (The