[R6RS] How SRFI 35 enables communication protocols via conditions

[Manuel Serrano]

> Both mechanisms are necessary, because it is not uncommon for several
> condition types not directly related via subtyping to apply to a given
> exceptional situation.  Two examples:
> 
> - File I/O errors that are also networking errors.
> - Errors that are also or may be bugs.
I have never faced situations where I had to compose exceptions (I don't
pretend that these do not exist) so I'm not absolutely convinced that this
is mandatory. 

We are going to add records to R6Rs, I think that exceptions must be
related to these. More precisely, I think that it is desirable to
have exceptions specified in terms of records.

-- 
Manuel