User blog comment:Wythagoras/Dollar function formal definition/@comment-5529393-20130616091711/@comment-10429372-20130616115049

Oops, my bad! a$[0]_[0][0] = a$[0]_a[0] = a$[[0]_a[0_(a-1)[0]]_(a-1)[0]...]_(a-1)[0] with a nests. Use rule 5