User blog comment:Wythagoras/Dollars Function/@comment-11227630-20130928111712/@comment-11227630-20130929055917

2. If I follow the rules, a$[b]2 expands to a$...[[b-1]2]... with a nests of []'s. But in 3$[[0]3]2, the b is [0]3. So I don't know which rule can I use.

3. If I follow your "what to solve first", I should first solve the [0] in the 3$[1][0] rather than [1][0] because [0] has level of 1, [1][0] has level of [0] and [0] has lower level than [1][0].