Now that @ may begin an identifier, we should probably make some comment about unquoting, i.e. to distinguish whether ,@foo is (unquote @foo) or (unquote-splicing foo).
Since these identifiers did not exist before, I recommend the latter (that is, use unquote-splicing if ,@ is seen), and we should add a note that if you want to unquote an identifier beginning with add you need to either insert whitespace or escape the identifier, e.g. , @foo or ,|@foo|.
The good news now is that SXML is no longer invalid syntax.