OnItem()

         Reactivates an item of a submenu

    Syntax:

         OnItem( aMenu, nSubMenu, nPos ) -> Nil

         aMenu    : The menu created with NewMenu()
         nSubMenu : The number of the submenu in which to reactivate the item
         nPos     : The position of the item in the submenu

    Description:

         Reactivates a menu item, making it selectable again.
         A divider line cannot be made selectable.
         Only an item previously deactivated can be reactivated.

    Return:

         Nil.

    Example:

         aMenu := NewMenu( { "^System", "^Tools", "^Other" }, ;
                           { "N/W", "R/W", "W/B",  "W+/B", ;
                             "N/W", "W/B", "N+/W", "R/W", "GR+/B" } )
         AddItem ( aMenu, 1, "Item ^1", "Message 1" )
         AddItem ( aMenu, 1, "Item ^2", "Message 2" )

         OffItem ( aMenu, 1, 2 )  // Deactivate the second item
         OnItem ( aMenu, 1, 2 )   // Now reactivate it