Iterative function f(n) from SICP exercise 1.11

sicp_ex_1.11.scm

; f(n) = n if n < 3,
; f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n > 3
;
; iterative (via tail recursion optimisation) 

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

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

From a discussion of the solution to SICP exercise 1.11 at http://www.enigmaticape.com/blog/sicp-exercise-1-11-iterative-function-fn/