User blog comment:Wythagoras/Strength of Hierarchical Nested Subscript Array Notation/@comment-5029411-20130731133113/@comment-10429372-20130731143220

Bird hasn't defined this in Beyond Nested Arrays IV old, but it can be a nice extension with a very high grow rate. it is similiar to the universal bracket type of HAN.

Also, things like chi(M(1)) don't exist. You can look for the good definitions of this here

This has a good chance to reach the limit of the compact ordinal.