code: @Action(
              semantics = SemanticsOf.SAFE
      )
      @MemberOrder(sequence = "3")
      public List<FullCalendar2WicketToDoItem> complete() {
          final List<FullCalendar2WicketToDoItem> items = completeNoUi();
          if(items.isEmpty()) {
              container.informUser("No to-do items have yet been completed :-(");
          }
          return items;
      }
docs: region > complete (action)