[plt-scheme] redefinition with #lang scheme

Hi,
I know that the following code works on the REPL, but why/how it should be done to work with #lang scheme ?
(define old-car car)
(define car (λ (lst)   (if (null? lst)  '()  (old-car lst))))
(car '())(car '(a b  c))
Thanks. 		 	   		  
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