Integer factorization in Chez Scheme

factor.scm

(define factor
  (lambda (n)
    (let ([next (lambda (i)
                     (if (zero? (modulo i 2))
                         (1+ i)
                         (+ 2 i)))])
      (let f ([n n] [i 2])
        (let ([d (/ n i)] [s (sqrt n)])
          (cond
           [(>= i s) (list n)]
           [(integer? d) (cons i (f d i))]
           [else (f n (next i))]))))))