I didn't say mature I said they were a good format for it. And by what metric are you measuring maturity?

   * Age? s-expressions predate python by a fair amount.
   * Parsers? just about every language I know has a lisp 
     interpreter embedded in it. s-expressions therefore    
     have 
     parsers in pretty much all languages.
   * Market Share? well xml and json may win there but I'm 
     not sure that's a valid reason to discount
     s-expressions