code: public void addAll(Set<? extends E> set) {
  		if(set.size()<=MAXIMUM_COMBINE_SIZE) {
  			if(combined==null) combined = new HashSet<>(set);
  			else combined.addAll(set);
  		} else {
  			added.add(set);
  		}
  	}
docs: If the set has size > MAXIMUM_COMBINE_SIZE, the set will be added to the
  <code>added</code> list, which may then be later combined only when needed.
  Because of this potentially delayed combining, any set added should not be
  subsequently altered.