Modifying nested dictionary/alist type data

[Paul Morris]

Hi, I’m rewriting one of LilyPond's C++ routines in Guile/Scheme in order to 
customize it for my own use, and it uses some nested dictionary/map/alist-style 
data structures that you can modify using something like this in C++:

  mydata[aaa][bbb].ccc = 10;

Ok, maybe use alists, but how to modify the nested values? ...given that you 
need an extra “set!” operation to do this reliably, as shown in the 
documentation:

(set! address-list
      (assoc-set! address-list "bob" "11 Newington Avenue"))

But if you have nested alists like this

  (define my-alist ‘((aaa . ((bbb . ((ccc . 10)))))))

and you want to change ccc, then the “set!” trick doesn’t help. (...what would 
you “set!”?)  So what’s the best approach here for such nested data structures? 
 Maybe I should just use nested hash maps instead?  Sorry if I’m missing 
something obvious.

Thanks,
-Paul