(define (foo xs)
(cond ((null? xs) ()) ; Base case 1: The empty list
((null? (cdr xs)) (cons (* 2 (car xs)) '())) ; Base case 2: The singleton
(else (cons (* 2 (car xs)) (cons (cadr xs) (foo (cddr xs))))))) ; Anything else
(display (foo '(3 4 5 6 7)))
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