}

  public void testRetainFrom() {
    assertEquals("aaa", is('a').retainFrom("bazaar"));
    assertEquals("z", is('z').retainFrom("bazaar"));
    assertEquals("!", is('!').retainFrom("!@#$%^&*()-="));
    assertEquals("", is('x').retainFrom("bazaar"));
    assertEquals("", is('a').retainFrom(""));
  }

  public void testPrecomputedOptimizations() {

  }

  public void testRetainFrom() {
    assertEquals("aaa", is('a').retainFrom("bazaar"));
    assertEquals("z", is('z').retainFrom("bazaar"));
    assertEquals("!", is('!').retainFrom("!@#$%^&*()-="));
    assertEquals("", is('x').retainFrom("bazaar"));
    assertEquals("", is('a').retainFrom(""));
  }

  public void testPrecomputedOptimizations() {

     *
     * <p>ASCII characters in the part are expected to be valid per RFC 1035, with underscore also
     * being allowed due to widespread practice.
     */

    String asciiChars = CharMatcher.ascii().retainFrom(part);

    if (!PART_CHAR_MATCHER.matchesAllOf(asciiChars)) {
      return false;
    }

    // No initial or final dashes or underscores.

    if (DASH_MATCHER.matches(part.charAt(0))

   * Returns a string containing all matching BMP characters of a character sequence, in order. For
   * example:
   *
   * {@snippet :
   * CharMatcher.is('a').retainFrom("bazaar")
   * }
   *
   * ... returns {@code "aaa"}.
   */
  public String retainFrom(CharSequence sequence) {
    return negate().removeFrom(sequence);
  }

  /**