some solutions for MIT Press's "structure and interpretation of computer programs", I guess
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(define (f-recur n)
(cond ((< n 3) n)
(else (+ (f-recur (- n 1))
(* 2 (f-recur (- n 2)))
(* 3 (f-recur (- n 3)))))))
(define (f-iter n) (iter 0 1 2 n))
(define (iter a b c count)
(if (= count 0)
a
(iter b c (+ c (* 2 b) (* 3 a)) (- count 1))))
(display (f-recur 7))
(newline)
(display (f-iter 7))
(newline)