Hmm. Marc mentioned the ";;>" comments in Chibi-Scheme; they're used
like this:

;;> \section{Interface}
;;>
;;> \procedure{(show out [args ...])}
;;>
;;> The primary interface.  Analogous to CL's \scheme{format}, the first
;;> argument is either an output port or a boolean, with \scheme{#t}
;;> indicating \scheme{current-output-port} and \scheme{#f} indicating a
;;> string port.  The remaining arguments are formatters, combined as with
;;> \scheme{each}, run with output to the given destination.  If \var{out}
;;> is \scheme{#f} then the accumulated output is returned, otherwise
;;> the result is unspecified.

So latex-like syntax (specifically, Scribble?) embedded in Scheme.

This could be fed to a Scheme procedure to get an equivalent SXML
representation, just like "#! Encoding: UTF-8" could be fed to a
procedure to parse it.

In both of these the idea is "embed foreign syntax into a Scheme file,
and feed it into a parser that converts it into an S-expression". This
is like a pass that comes before Scheme's usual macroexpansion pass,
turning the file into something that is all S-expressions.

The Scribble markup could be written:

#! Scribble:
#! \section{Interface}
#!
#! \procedure{(show out [args ...])}
#!
#! The primary interface.  Analogous to CL's \scheme{format}, the first
#! argument is either an output port or a boolean, with \scheme{#t}
#! indicating \scheme{current-output-port} and \scheme{#f} indicating a
#! string port.  The remaining arguments are formatters, combined as
#! End:

I.e. some way to embed line-oriented external syntax like ";" comments,
except that they're not ignored like comments are.

And some way to mix different external syntaxes and work out what's
what. "\section{Interface}” is Scribble syntax but "encoding: utf-8" is
not. This is an open problem.