User blog comment:Wythagoras/Dollar function: final version/@comment-5529393-20131116082407/@comment-10429372-20131117170130

Remove zero, by rule 5.

But, I think that's not what you mean.

a$0]2] = a$[[[...] by rule 6 and

a$[[0]c] = a$[[[0]c]c-1] by rule S2 and a$[[[0]c]c-1] = a$[[[[...]c-1]c-1]c-1] by rule 6