code: public Set<ClassDescriptor> getSubtypes(ClassDescriptor classDescriptor) throws ClassNotFoundException {
          Set<ClassDescriptor> result = subtypeSetMap.get(classDescriptor);
          if (result == null) {
              result = computeKnownSubtypes(classDescriptor);
              subtypeSetMap.put(classDescriptor, result);
          }
          return result;
      }
docs: Get known subtypes of given class. The set returned <em>DOES</em> include
  the class itself.
  
  @param classDescriptor
  ClassDescriptor naming a class
  @return Set of ClassDescriptors which are the known subtypes of the class
  @throws ClassNotFoundException