[racket] illegal placeholder cycle

At Tue, 6 Sep 2011 16:21:57 -0400, Doug Orleans wrote:
> This error message is surprising:
> 
> > (shared ((c (append null c))) c)
> make-reader-graph: illegal placeholder cycle in value: '(#<placeholder>)
> 
> Is it unreasonable to expect this to evaluate to null?

I don't see how to get `null', although I could see an argument for
#<undefined>: since c = (append null c) for any c, the above seems the
same as

 (shared ([c c]) c)

to me.