[plt-scheme] namespace question

(define ns/set! namespace-set-variable-value!)

; as per help desk: no problems
(define x 'orig)
(let ([n (make-namespace)])
   (parameterize ([current-namespace n])
     (eval '(define x 'new))
     (eval '(display x))
     (display x)))

prints out: neworig

but :

(let ([n (make-namespace)])
   (parameterize ([current-namespace n])
     (ns/set! y 'wft)
     (eval '(display y))))

results in : "reference to undefined identifier: y"

so, obviously, I don't understand namespace-set-variable-value!
enlightenment (or, rather, an explanation... I may never achieve 
enlightenment...:) would be appreciated

Dr. Chris Wright
Medical Director
Intensive Care Unit
Monash Medical Centre, Clayton VIC