docs: public int compare(final SnomedConcept o1, final SnomedConcept o2) {
          if (o1 == o2) return 0;
          if (o1 != null) {
              if (o2 == null) return 1;
                  // sort shorter mapping groups first
              else if (o1.mappingGroup.length() != o2.mappingGroup.length()) {
                  return ((Integer)o1.mappingGroup.length()).compareTo(o2.mappingGroup.length());
              }
              else return o1.mappingGroup.compareTo(o2.mappingGroup);
          }
          return -1;
      }