(define (phi x)
; constants
(let* ((a1 0.254829592)
(a2 -0.284496736)
(a3 1.421413741)
(a4 -1.453152027)
(a5 1.061405429)
(p 0.3275911)
; Save the sign of x
(sign (if (< x 0) -1 1))
(a (/ (abs x) (sqrt 2.0)))
; A&S formula 7.1.26
(t (/ 1.0 (+ 1.0 (* p a))))
(y (- 1.0 (* (+ (* (+ (* (+ (* (+ (* a5 t) a4) t) a3) t) a2) t) a1) t (exp (* (- a) a))))))
(* 0.5 (+ 1.0 (* sign y)))))