The SRFI fails to distinguish between sharing and circularities. This is
important because R5RS *does* specify how non-circular shared structures
should be written/displayed(*). For instance
  (define x (cons #f #f))
  (define y (cons 1 1))
  (set-car! x y)
  (set-cdr! x y)
  (write x)
should produce
  ((1 . 1) 1 . 1)
when written in an R5RS-conformant system, and *not*
  (#0=(1 . 1) . #0#)

iirc, Al Petrofsky posted an algorithm that detects circularities
without also catching sharing. It has higher computational complexity
and is harder to implement than detecting all sharing.

CL makes this distinction. See
   http://www.lispworks.com/reference/HyperSpec/Issues/iss274_w.htm

(*) Arguably, R5RS even specifies how circular structures should be
written - as infinite outputs.

Matthias.