Using standard @ instead of ^ as the SXML attributes symbols.
This was never a portable aspect of SXML before because in earlier
standards @ was an illegal identifier, but it is allowed in R7RS.
Scribble requires escaping with @|@| to pass through a raw @ symbol.
Considering using \ as the default scribble escape instead.
(chibi mime) now case-folds then interns headers to match normal SXML.