[plt-scheme] let-syntax is now implemented in r5rs language?

Hi all,

I'm using plt-scheme 204, the example about let-syntax in the standard
returns an error in plt-scheme 204 using r5rs language:

Welcome to DrScheme, version 204.
Language: Standard (R5RS).
> (let-syntax ((when (syntax-rules ()
                       ((when test stmt1 stmt2 ...)
                        (if test
                            (begin 
                              stmt1
                              stmt2
                              ...))))))
    (let ((if #t))
      (when if 
        (set! if 'now))
      if))
. reference to undefined identifier: let-syntax
> 

Why?

Best regards,

Paulo J. Matos