set-empty-callbacks

Sets the empty callbacks. These functions are called when the frame group is empty.

• (send a-group: set-empty-callbacks test close-down) -> void
      test : (-> boolean)
      close-down : (-> void)

  The function test is called when there is one frame left in the group. If it returns #t, the closing operation may be completed. If it returns #f, the closing operation is aborted.

  The function close-down is called when there are no frames left in the group.