Discovering Format Directives

So I was just reading some Common Lisp information on the web when I saw these really cool code examples in Peter Siebel's excellent book "Practical Common Lisp" that uses format. I have used format but had not realized how cool it really is.

The first very interesting example used the "~{" and "~}" directive which lets us use nested lists as parameters to format. I decided to test this on my own for an understanding

[30]> (format t "~{~{~a~%~a~%~}~}" '((a b)))

A

B

NIL

Looks good. So I tried

[34]> (format t "~{~{~a~%~a~%~}~}" '((a b) (c d) (e f) (g h)))

A

B

C

D

E

F

G

H

NIL

Even better. So if it supports nested lists, then it should obviously allow more nesting and so I tried

[35]> (format t "~{~{~{~a~%~a~%~}~}~}" '(((a b))))

A

B

NIL

[36]> (format t "~{~{~{~a~%~a~%~}~}~}" '(((a b) (c d))))

A

B

C

D

NIL

Yup. As usual in impressive Lisp style, my REPL responds as expected. So I was pretty happy about learning something new, thanks to Peter Siebel, I read further down and finally ended up in the footnotes section where I found this gem.

[37]> (format nil "~r" 1606938044258990275541962092)

"one octillion, six hundred and six septillion, nine hundred and thirty-eight sextillion, 

forty-four quintillion, two hundred and fifty-eight quadrillion, nine hundred and ninety 

trillion, two hundred and seventy-five billion, five hundred and forty-one million, nine 

hundred and sixty-two thousand, ninety-two"

Holy mother of batman! That is bloody impressive for a function I have so far only used for printing. Note to self -> ALWAYS READ FOOTNOTES. Anyway, curious about how much it would take, I figured that Clisp can take a max of One Hundred Vigintillion which is 10^66. So guess what I had to know all the intermediate names.

[38]> (do ((x (expt 10 9) (* x 1000)))

    ((>= x (expt 10 66)))

  (format t "~r = ~a~%" x x))

one billion = 1000000000

one trillion = 1000000000000

one quadrillion = 1000000000000000

one quintillion = 1000000000000000000

one sextillion = 1000000000000000000000

one septillion = 1000000000000000000000000

one octillion = 1000000000000000000000000000

one nonillion = 1000000000000000000000000000000

one decillion = 1000000000000000000000000000000000

one undecillion = 1000000000000000000000000000000000000

one duodecillion = 1000000000000000000000000000000000000000

one tredecillion = 1000000000000000000000000000000000000000000

one quattuordecillion = 1000000000000000000000000000000000000000000000

one quindecillion = 1000000000000000000000000000000000000000000000000

one sexdecillion = 1000000000000000000000000000000000000000000000000000

one septendecillion = 1000000000000000000000000000000000000000000000000000000

one octodecillion = 1000000000000000000000000000000000000000000000000000000000

one novemdecillion = 1000000000000000000000000000000000000000000000000000000000000

one vigintillion = 1000000000000000000000000000000000000000000000000000000000000000

NIL

I had a "for" loop macro construct that I used to use for simple stuff but realized I need to modify it after this example and had to get into a "DO". I would appreciate if someone can show me how to do this with the "LOOP" macro.