A few questions/comments:

1)
Are parameters a new data type? This srfi does not define any procedures
on parameters, so in that sense they are not. However, implementations
may want to define such procedures, in which case having a predicate
|parameter?| would be essential. Note however that r5rs compliance would
still require |procedure?| to return #t for parameters.

2)
What happens when the converter procedure returns more than once, e.g.
when a continuation captured inside it is invoked?

3)
It would be useful for (<parameter> <value>) to return the existing
value since a lot of uses of dynamic parameters are of the pattern:
  * get parameter value -> old_value
  * set parameter value
  * do something
  * set parameter value to old_value
Having a setter that returns the existing value allows elimination of
the first step.

Matthias.