Set the selection between index and the anchor point. If index is before the anchor point, the selection runs from index up to but not including the anchor point; if it is after, the selection runs from the anchor point up to but not including index; if it is the same, nothing happens. The anchor point is the one set by the most recent selection_from() or selection_adjust() call. If the selection is not in this widget, a new selection is created using the most recent anchor point.