Exercise 1.11.  A function f is defined by the rule that f(n) = n if n<3 and 
f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n≥3. Write a procedure that computes 
f by means of a recursive process. Write a procedure that computes f by means 
of an iterative process.

Recursive process:

(define (f n)
  (if (n < 3)
      n
      (+    (f (- n 1))
         (* (f (- n 2)) 2)
         (* (f (- n 3)) 3))))

Iterative process:

(define (f n)
  (if (n < 3)
      n
      (f-iter 0 1 2 (- n 2))))

(define (f-iter a b c n)
  (if (= n 0)
      c
      (f-iter b c (+ c
                     (* 2 b)
                     (* 3 a)) (- n 1))))