docs: public String stack() {
      if (d1 <= 0) {
        // No stack, detail or sequence is present
        return null;
      } else if (d2 < 0) {
        // Stack, but no detail is present
        return substr(asg, d1 + 1, dN);
      } else {
        // Stack and at least one of detail or sequence is present
        return substr(asg, d1 + 1, d2);
      }
    }