code: private SortedSet<ClassDoc> get(Map<ClassDoc, SortedSet<ClassDoc>> map, ClassDoc cd) {
          SortedSet<ClassDoc> aset = map.get(cd);
          if (aset == null) {
              return new TreeSet<>(comparator);
          }
          return aset;
      }
docs: From the map return the list of sub-classes or sub-interfaces. If list
  is null create a new one and return it.
  
  @param map The entire map.
  @param cd class for which the sub-class list is requested.
  @returns List Sub-Class list for the class passed.