User blog comment:Mh314159/new YIP notation/@comment-37246647-20190712002005/@comment-39585023-20190712011140

When you reach f1 8 (8) you just recurse the f1 function 8 times, so you get f1(f1...f1(8) with 8 copies of f1.  Thanks for pointing out that I should be more specific that superscripts indicate functional recursion; it's a pretty standard thing but it's better if I'm very clear.