@Override
    public NavigableSet<E> headSet(@ParametricNullness E toElement, boolean inclusive) {
      return filter(unfiltered().headSet(toElement, inclusive), predicate);
    }

    @Override
    public NavigableSet<E> tailSet(@ParametricNullness E fromElement, boolean inclusive) {
      return filter(unfiltered().tailSet(fromElement, inclusive), predicate);
    }
  }

  /**

   * is fine with that.
   */

  /**
   * Ensures that {@code index} specifies a valid <i>element</i> in an array, list or string of size
   * {@code size}. An element index may range from zero, inclusive, to {@code size}, exclusive.
   *
   * @param index a user-supplied index identifying an element of an array, list or string
   * @param size the size of that array, list or string
   * @return the value of {@code index}

    return anyOf(sequence).negate();
  }

  /**
   * Returns a {@code char} matcher that matches any character in a given BMP range (both endpoints
   * are inclusive). For example, to match any lowercase letter of the English alphabet, use {@code
   * CharMatcher.inRange('a', 'z')}.
   *
   * @throws IllegalArgumentException if {@code endInclusive < startInclusive}
   */