2.20

(define (same-parity . lst)  (define is-odd (odd? (car lst)))  (define (proc l)    (if (null? l)        l        (let ((x (proc (cdr l)))              (y (car l)))          (if (equal? (odd? y) is-odd)              (cons y x)              x))))  (proc lst))

2.21

(define (square-list items)  (if (null? items)      items      (cons (expt (car items) 2)            (square-list (cdr items)))))(define (square-list2 items)  (map (lambda (x) (* x x)) items))

2.23

(define (for-each f lst)  (if (null? lst)      #t      (cond (else (f (car lst))                  (for-each f (cdr lst))))))