User blog comment:Wythagoras/Dollar Function: new version/@comment-25418284-20131023235926/@comment-3427444-20131024025757

They are used when a bracket is surrounded with lower level brackets. For example:
 * 3$[[0]2] = 3$[0]2[0]2[0]2
 * 3$[[0]4]3 = 3$[[[[0]4]2]2]2