code: private Collection<ClassDoc> subclasses(ClassDoc cd) {
          Collection<ClassDoc> ret = classToSubclass.get(cd.qualifiedName());
          if (ret == null) {
              ret = new TreeSet<>(utils.makeComparatorForClassUse());
              SortedSet<ClassDoc> subs = classtree.subclasses(cd);
              if (subs != null) {
                  ret.addAll(subs);
                  for (ClassDoc sub : subs) {
                      ret.addAll(subclasses(sub));
                  }
              }
              addAll(classToSubclass, cd, ret);
          }
          return ret;
      }
docs: Return all subclasses of a class AND fill-in classToSubclass map.