code: public void addExtendRelationship(TemplateClass parent, TemplateClass child) {
          if (mode().isProd()) return;
          Set<TemplateClass> children = extendMap.get(parent);
          if (null == children) {
              children = new HashSet<TemplateClass>();
              extendMap.put(parent, children);
          }
          children.add(child);
      }
docs: Not an API for user application
  
  @param parent
  @param child