(let ((a (list 1 2)) (b (list 1 2)) (c (list 3 4))) (append a b c)) <==> (let ((a (list 1 2)) (c (list 3 4))) (append a a c)) <==> (let ((a (list 1 2))) (append a a (list 3 4))) <==> (let ((a (list (- 3 2) 2)) (b (list 1 (+ 1 1)))) (append a b (list 3 4))) <==> (list 1 2 1 2 3 4)