This SRFI has a lot of good in it, but I'm glad the thread-specific
aspects are being treated separately.

A minor suggestion. One of the nice things about Scheme is it's
relatively uniform names. With this in mind, I'd propose:

   (make-parameter initial-value converter) => parameter
   (set-parameter! parameter value)

Then, as Olin Scrivers suggested, add the library procedure and derived
syntax:

   (with-parameter parameter value thunk)
   (let-parameter ((parameter value) ...) body)
   (let-parameter* ((parameter value) ...) body)

When you do deal with threads, please be careful when using the phrase
"mutable parameter", as different types of parameter can be "mutable"
with different semantics (for example, globally shared, shared between
parents and children, not shared). Please use a more specific term, like
thread-local, depending on the exactly what you mean.

Regards,

Alan
--
Dr Alan Watson
Instituto de Astronomía UNAM