the items may be selected at once. Multiple selections, as in the extended-selection GenItemGroup, are stored as lists of items. Selecting an already selected item will deselect that item. Therefore, there may be a state where nothing is selected within a non-exclusive list.</P>
<P>
For example, an interactive tutorial may ask the user if he wants more information on certain topics, giving these topics in a non-exclusive list. The user can freely choose as many or as few items as he wishes, and then select 