[racket] turning off shared printing in pconvert

This program

#lang racket
(require mzlib/pconvert)
(parameterize ([show-sharing false])
  (print-convert
   (shared ([-a- (list 0 -b-)]
            [-b- (list 1 -a-)])
     -a-)))

produces this output in 5.1.3:

Language: racket; memory limit: 128 MB.
'(shared ((-0- `(0 (1 ,-0-)))) -0-)

How do I get pconvert to suppress sharing?

(The only option I see right now is to supply #t as the second
argument to print-convert, so that it suppresses sharing for DAGs and
only shows true cycles.)

Shriram