code: IConceptSet get(int concept) {
          IConceptSet subsumes = set.get(concept);
  
          if (null == subsumes) {
              // A Concept always subsumes itself and TOP
              subsumes = new SparseConceptSet();
              subsumes.add(concept);
              subsumes.add(IFactory.TOP_CONCEPT);
              set.put(concept, subsumes);
              size++;
          }
  
          return subsumes;
      }
docs: /*
  (non-Javadoc)
  
  @see au.csiro.snorocket.Subsumptions#get(int)