Systematic examination of XML, JSON, YAML, JSX, Tcl, Rebol, and Forth against the six roles SX requires (markup, language, wire format, data notation, spec language, metaprogramming). Comparison table across five properties. Every candidate either fails requirements or converges toward s-expressions under a different name. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>