> I don't understand this. You cite the R6RS condition hierarchy, which is
> actually quite simple. This is defined through a number of libraries,
> which export things like `assertion-violation?' or `i/o-encoding-error?'
> so this is exactly what you propose. Any other library providing I/O
> could export its own `i/o-encoding-error?' that is not related to R6RS's.

That would work fine, but R6RS also has record inspection, which lets
you find out the precise type of the record. IIRC the standard
exceptions are raised as condition objects which are a kind of
inspectable record. User code could rely on the exactly right exception
type being raised, instead of just checking whether it matches a predicate.