Port position for transcoded port

I'm planing to support port-position and set-port-position! for transcoded textual ports and checked some major R6RS implementation how they act. The result was rather interesting.

First of all, I write the implementations I checked and its result. (I don't put Sagittarius because it's obviously not supporting it yet :-P)

Petite Chez 8.4
Larceny 0.9.7

Error: no handler for exception #<record &compound-condition>
Compound condition has these components:
#<record &assertion>
#<record &who>
    who : set-port-position!
#<record &message>
    message : "position not obtained from port-position"
#<record &irritants>
    irritants : (#<INPUT PORT test.txt> 2)

Terminating program execution.
Mosh 0.2.7
Ypsilon 0.9.6-update3
Racket 5.2.1
The test code;
(import (rnrs))

(call-with-input-file "test.txt"
  (lambda (p)
    (display (port-has-port-position? p)) (newline)
    (display (port-has-set-port-position!? p)) (newline)
    (when (port-has-port-position? p)
      (get-char p)
      (display (port-position p)) (newline)
      (set-port-position! p 2)
      (display (get-char p)) (newline))))
test.txt (UTF-8)
Mosh doesn't support the port-position so test was skipped.Except Larceny, the other implementations simply set the position of underlying binary port. So it returned the invalid character. On the other hand, Larceny is checking the position of textual port and if it mismatches then raises an error. (although, if I change the setting position to 0, then it reads an invalid character, so seems not really working.)

I'm not sure which is the expected behaviour but at least the way Chez, Ypsilon and Racket are doing is easy enough to implement.

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