docs: public Optional<Character> character() {
      char c = 0;
      int count = 0;
      for (int i = 0; i < members.length; ++i) {
        if (members[i]) {
          c = (char) i;
          ++count;
        }
      }
      return (count == 1) ? Optional.of(c) : Optional.empty();
    }