code: public void addElement(Element element) {
  		synchronized(lock) {
  			checkNotFrozen();
  			element.setPage(this);
  			// elements
  			if(elements == null) elements = new ArrayList<>();
  			elements.add(element);
  			// elementsById
  			addToElementsById(element, false);
  		}
  	}
docs: Adds an element to this page.