±d6 in Guile Scheme

Just for fun: This implements the ±d6 in Guile Scheme, including critical rolls.

(define* (roll-1d6)
  (let* ((numbers '(-5 -3 -1 2 4 6))
         (rolld6
          (lambda () (list-ref
                      numbers
                      (random 6)))))
    (let rolling ((rolled
                   (cons (rolld6) '())))
      (cond
       ((and (= 1 (length rolled))
             (member
              (car rolled) '(-5 6)))
        (rolling (cons (rolld6) rolled)))
       ((and (> (length rolled) 1)
             (equal? (car rolled)
                     (car (cdr rolled))))
        (rolling (cons (rolld6) rolled)))
       ((= 1 (length rolled))
        (car rolled))
       (else
        (apply + (cdr rolled)))))))

Written for syncbot, an IRC bot for 8sync.