code: private static String generateGroupElement(String id, List<Double> list) {
  		StringBuilder sb = new StringBuilder();
  
  		if (list.size() == 1) {
  			if (list.get(0) == -1.0) {
  				sb.append(id + ":");
  				sb.append("?" + "-");
  			} else if (list.get(0) == 1.0) {
  				sb.append(id + ":");
  			} else {
  				sb.append(id + ":" + list.get(0) + "-");
  			}
  		} else {
  			sb.append(id + ":");
  			for (Double item : list) {
  				sb.append(item + "-");
  			}
  		}
  
  		sb.setLength(sb.length() - 1);
  		return sb.toString();
  	}
docs: method to generate the MonomerNotationGroupElement in String format
  
  @param id
  @param list
  @return MonomerNotationGroupElement in String format