docs: private void primAddMember(IGroupMember gm) throws GroupsException {
          final EntityIdentifier cacheKey = getUnderlyingEntityIdentifier();
          Element element = childrenCache.get(cacheKey);
  
          @SuppressWarnings("unchecked")
          final Set<IGroupMember> set =
                  element != null ? (Set<IGroupMember>) element.getObjectValue() : buildChildrenSet();
  
          final Set<IGroupMember> children = new HashSet<>(set);
          children.add(gm);
          childrenCache.put(new Element(cacheKey, children));
      }