But I don't want to change what Guile has at the moment. I think
Emacs syntax classes and Perl's corresponding (simpler) extensions are
nice, but Guile uses whatever POSIX facilities it can find already
present on the system, which I think is the right thing for now. This
means we don't really have the option of changing the syntax, except
as a pre-processing pass, which can be implemented as a module, and
doesn't require changes to the core.