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-struct node (ssn name left right)) (define unique1* (make-node 5 'Tingting false false)) (define node1 unique1*) (define unique2* (make-node 1 'James false node1)) (define node2 unique2*) (set-node-ssn! node1 6)