User blog comment:Wythagoras/Strength of Hierarchical Nested Subscript Array Notation/@comment-7484840-20130730134356/@comment-7484840-20130801164937

I think I've already got that, with a very loosely defined type on the angle brackets which is within square brackets with quill (⁅ and ⁆), which I believe take it a long way further, although to be honest I have no idea how much further. An array in them might work a bit like the array after an I in the brackets, and could possibly take it as far as the limit of Mahlo.