Determine the greatest common divisor of two positive integer numbers

	;; P32 (**) Determine the greatest common divisor of two positive integer numbers.
	;; Use Euclid's algorithm.
	;; Example:
	;; * (gcd 36 63)
	;; 9
	(define (gcd a b)
	(let loop ((x a) (y b) (z 1) (w 0))
	(let ((q (quotient x y)) (r (modulo x y)))
	(if (zero? r)
	y
	(loop y r w (- z (* q w)))))))

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