[MIT-Scheme-users] SICP ex. 1.16

[antiloquax]

Hi, I am learning Scheme using SCIP and R. Kent Dybvig's "The Scheme
Programming Language".
I am stuck on exercise 1.16 in Abelson and Sussman.
The task is to "design a procedure that evolves and iterative exponentiation
process that uses successive squaring".

Well, I can't seem to get it working.
The code I have tried looks something like this:
[code]
; iterative process for fast exponent
; NOT WORKING!!!

(define (expt b p)
  (exp-it b p 1))

(define (exp-it b p a)
  (cond 
    ((= p 0) a)
    ((even? p) (exp-it b (/ p 2) (* a (square b)))))
    (else (exp-it b (- p 1) (* b a)))))
[/code]

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