He is kind of right, since he is talking a about extending the basic XML _syntax_, not about creating or extending XML vocabularies. Obviously his solution with

He is kind of right, since he is talking a about extending the basic XML _syntax_, not about creating or extending XML vocabularies. Obviously his solution with reader macros only works in Lisp, and he seem to be proud about that, but it kind of defeats the purpose of a platform-independent exchange format.