docs: public int compare(final SnomedConcept o1, final SnomedConcept o2) {
          if (o1 == o2) return 0;
          if (o1 != null) {
              if (o2 == null) return 1;
                  // need to use BigInteger to compare SNOMED codes since some are fantastically long
              else return new BigInteger(o1.snomedId)
                      .compareTo(new BigInteger(o2.snomedId));
          }
          return -1;
      }