code: public boolean defineElement(String name, Node definingNode) {
      elements.add(name);
      return defineDeclaredProperty(name, elementsType, definingNode);
    }
docs: Defines a new element on this enum.
  @param name the name of the new element
  @param definingNode the {@code Node} that defines this new element
  @return true iff the new element is added successfully