extras = new ArrayList<>(MAX_EXTRAS);
        extras.add(o);
      } else if (extras.size() < MAX_EXTRAS) {
        extras.add(o);
      } else {
        throw multiples(true);
      }
    }

    ToOptionalState combine(ToOptionalState other) {
      if (element == null) {
        return other;
      } else if (other.element == null) {
        return this;
      } else {