> I don't know whether it's still around on the Net somewhere, but
> several years ago someone posted a file named xii.tex
> When processed under plain TeX it output the familiar "Twelve Days of
> Christmas".

ctan macros/plain/contrib/xii.tex

> But the author, whose name I've forgotten but was a well-known
> TeXpert, had managed to conceal all this under a maze of obscurity.

david carlisle.  he didn't want it preserved for posterity, but i
persuaded him to let me put it on ctan.

> Anyone wanting to try their hand at writing something to parse TeX
> should see how their program works on xii.tex

i can hardly parse the thing as a gendanken-experiment, using my own