The part of the code marked in green is the part which is about to be expanded, and the part marked in red is the part which was just expanded.
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(define unique1* (cons 'c empty)) (define unique2* (cons 'b unique1*)) (define lst-1 unique2*) (define lst-2 (cons 'a unique1*)) (set-rest! (rest lst-2) (cons 'd empty)) (set! lst-1 (cons 'a (cons 'd empty))) (set-rest! lst-2 (rest (rest lst-2))) (set-rest! lst-1 empty)