code: public final void traverse(ZooPC pc) {
      	try {
  	    	traverseObject(pc);
  	        traverseWorkList();
         	} finally {
      		workList.clear();
      		toBecomePersistent.clear();
              //We have to clear the seenObjects here, see also issue #58.
              seenObjects.clear();
      	}
      }
docs: This can be used to enforce traversal of an object even if it is PERSITENT_CLEAN.
  This can be necessary for objects that transition from DETACHED_CLEAN
  to PERSITENT_CLEAN and whose children need to make this transition transitively.
  @param pc Object to check.