code: protected final Iterator buttonIterator() {
  
  		if (this.faceButtonManagers == null)
  			return Collections.EMPTY_SET.iterator();
  
  		return new NestedButtonIterator(this.faceButtonManagers.values().iterator());
  	}
docs: Returns an iterator over <em>all</em> buttons by traversing
  <em>each</em> {@link CommandFaceButtonManager}.