[racket] recognizing undefined; shared in TR

On Sat, Sep 3, 2011 at 11:17 PM, Shriram Krishnamurthi <sk at cs.brown.edu> wrote:
> Is there a way of checking whether a value position is undefined?
> Specifically, I'm trying to recognize the undefined value put in
> structures by SHARED.  It appears that a undefined value is sometimes
> EQ? to another one, but this doesn't seem to be specified in the
> documentation (ie, that there is only one undefined value).

Anyone have any thoughts on this?  Is it okay to use EQ?