code: @Override
  	public boolean addAll(Collection<? extends E> c) {
  		if(c.isEmpty()) return false;
  		combine();
  		if(combined==null) {
  			combined = new HashSet<>(c);
  			return true;
  		} else {
  			return combined.addAll(c);
  		}
  	}
docs: Triggers combining.
  
  @see  #addAll(java.util.Set)