[plt-scheme] Generate fresh variables in REDEX metafunctions?

Hi, all:

     It seems the following does not work

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========================================================================
(define-metafunction refT
     basic-grammar
     [(((T_4 x_4) ... (ptype_1 x_1) (T_2 x_2) ...) (x_1 x_3 ...))
      (CH (((ptype_1 &) x_1 boxed_1) (refT (((T_4 x_4) ... (ptype_1  
x_1) (T_2 x_2) ...) (x_3 ...)))))
      (side-condition (term (fresh boxed_1)))]
     [(((T_4 x_4) ... (T_1 x_1) (T_2 x_2) ...) (x_1 x_3 ...))
      (CH ((T_1 x_1 x_1) (refT ((T_4 x_4) ... (T_1 x_1) ((T_2  
x_2) ...) (x_3 ...)))))
      ]
     [(((T_1 x_1) ...) ()) ()])

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========================================================================

I just want to generate the fresh variables like  
boxed_1 ,boxed_2 ..... in the meta-function not in the reduction rules?

How can I ? or the redex version is too old?

thanks



Gang






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