}

  /**
   * Implemented by the user to determine whether {@code a} and {@code b} are considered equivalent,
   * subject to the requirements specified in {@link #equivalent}.
   *
   * <p>This method should not be called except by {@link #equivalent}. When {@link #equivalent}
   * calls this method, {@code a} and {@code b} are guaranteed to be distinct, non-null instances.
   *
   * @since 10.0 (previously, subclasses would override equivalent())