code: @Override
  	@SuppressWarnings("unchecked")
  	public boolean addAll(Collection<? extends E> c) {
  		if(c.isEmpty()) return false;
  		if(c instanceof Set) return addAll((Set<? extends E>)c);
  		else throw new UnsupportedOperationException("May only add sets");
  	}
docs: Must be a set.
  
  @see  #addAll(java.util.Set)