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.
This is the recommended syntax for error-handling in Chibi-specific
programs, since it's not possible to get stack traces when using
R[67]RS guard. guard is defined separately and according to the
standard in (scheme base).
