Non-upward-compatibility of Chez Scheme versions

[Olin.Shivers@CENTRO.SOAR.CS.CMU.EDU]

	   (let ([null? null?])
	     (define-macro! nil? (x)
	       `(,null? ,x))		; note comma before null?

I don't use the Scheme's you use, but I think I can see at least one
problem with this macro: functions are *not* defined in Scheme to be 
self-evaluating like, for example, integers are.
So instead of: 
    ,null?
you need:
    ',null?
As in:
	(let ((null? null?))
	  (define-macro! nil? (x)
	    `(',null ,x)))

    -Olin