S-exprs are a degenerate case of XML

<s-exp>defun foo <s-exp>x</s-exp>
<s-exp>let <s-exp>y <s-exp>1+ x</s-exp></s-exp></s-exp>
<s-exp>format t "y = ~d" y</s-exp></s-exp>

>>18
Well why doesn't anyone use Sexp in place of XML then, smartass? I'll tell you why: XML has schemas and validation, which isn't possible with Sexps.