[plt-scheme] mutable strings

We're trying to undo Scheme's fussiness about immutable strings.
More precisely, we're writing another language that macroexpands
into Scheme code, and in this language you can modify strings.

Is it possible to have a mutable string within a mzscheme function?

Because I'd considered, for example, hacking the reader so that
a double-quoted string was copied on being read in, and the copy,
which was therefore mutable, was used in the code.  But can you
even have a mutable string within code?

If anyone has any advice about how to do this, let me know.

Thanks, --pg