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

  /**