factorial

(defun factorial (n)

(cond ((or (not (integerp n))

(< n 0))

(error

"argument is not a non-negative integer"))

(t (fact n))))

;;; assumes a non-negative integer

;;; computes factorial of its argument

(defun fact (arg)

(if (zerop arg)

1

(* arg (fact (1- arg))))) ; recurse

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