[r6rs] frontier port position rejected with bytevector ports

[Marco Maggi]

The following program fails:

#!r6rs
(import (rnrs))
(define port (open-bytevector-input-port '#vu8()))
(set-port-position! port (port-position port))

and the following fails, too:

#!r6rs
(import (rnrs))

(define port (open-bytevector-input-port '#vu8(1 2 3)))

(write (port-position port))(newline)
(get-u8 port)
(write (port-position port))(newline)
(get-u8 port)
(write (port-position port))(newline)
(get-u8 port)
(write (port-position port))(newline)

(set-port-position! port (port-position port))

because of an attempt to set the port position to an invalid
value; but  if a position  is returned by  PORT-POSITION, it
must be  possible to SET-PORT-POSITION! to  such value.  All
the R6RS implementations I tested (but Mosh) accept this.
-- 
Marco Maggi