code: public int compareToPositional(ResidueNumber other) {
  		// sequence number
  		if (seqNum != null && other.seqNum != null) {
  			if (!seqNum.equals(other.seqNum)) return seqNum.compareTo(other.seqNum);
  		}
  		if (seqNum != null && other.seqNum == null) {
  			return 1;
  		} else if (seqNum == null && other.seqNum != null) {
  			return -1;
  		}
  
  		// insertion code
  		if (insCode != null && other.insCode != null) {
  			if (!insCode.equals(other.insCode)) return insCode.compareTo(other.insCode);
  		}
  		if (insCode != null && other.insCode == null) {
  			return 1;
  		} else if (insCode == null && other.insCode != null) {
  			return -1;
  		}
  
  		return 0;
  	}
docs: Compare residue numbers by sequence number and insertion code,
  ignoring the chain
  @param other
  @return