2.21

1 ]=>(define nil (list))

;Value: nil

1 ]=> (define (square x) (* x x)) 

;Value: square

1 ]=> (define (square-list items) 
       (if (null? items)
           nil
           (cons (square (car items))
                 (square-list (cdr items)))))

;Value: square-list

1 ]=> (square-list (list 1 2 3 4))

;Value 11: (1 4 9 16)

1 ]=> (define (square-list items)
       (map square items))

;Value: square-list

1 ]=> (square-list (list 1 2 3 4))

;Value 12: (1 4 9 16)