User blog comment:Wythagoras/Dollars function/@comment-1605058-20130707063642/@comment-10429372-20130707104434

About the subscript array: all brackets have the same a$.

For example: 3$[0]_[1] = 3$[0]_([0][0][0])

There is no rule for it, but is in the 'What to solve first' paragraph.