code: public T addElement(T element) {
  		// make sure that elements are initialized
  		getElements();
  		if (!containsElement(this.elements, element)) {
  			this.elements.add(element);
  			element.addChangeListener(this.elementChangeListener);
  			element.notifyState();
  			return element;
  		}
  		throw new RuntimeException(element.toString() + " already exists");
  	}
docs: add a new element, throw a RuntimeException if the element already exists
  @param element
  @return the added element