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(define (getopt defn msg args) ; => (values (list opt/arg ...) (list file ...))
  (define (parse-options defn)
    (let loop ((options (string->list defn)) (lones '()) (args '()))
      (cond ((null? options) (values lones args))
            ((null? (cdr options)) (values (cons (car options) lones) args))
            ((char=? (cadr options) #\:)
              (loop (cddr options) lones (cons (car options) args)))
            (else (loop (cdr options) (cons (car options) lones) args)))))
  (let-values (((lone-options arg-options) (parse-options defn)))
    (let loop ((args args) (xs '()))
      (cond ((or (null? args) (not (char=? (string-ref (car args) 0) #\-)))
              (values (reverse xs) args))
            ((string=? (car args) "--") (values (reverse xs) (cdr args)))
            ((member (string-ref (car args) 1) lone-options)
              (let* ((len (string-length (car args)))
                     (arg (string-ref (car args) 1))
                     (rest (substring (car args) 2 len)))
                (if (= len 2)
                    (loop (cdr args) (cons (list arg) xs))
                    (loop (cons (string-append "-" rest) (cdr args)) (cons (list arg) xs)))))
            ((member (string-ref (car args) 1) arg-options)
              (let* ((len (string-length (car args)))
                     (arg (string-ref (car args) 1))
                     (rest (substring (car args) 2 len)))
                (cond ((and (= len 2) (null? (cdr args))) (error 'getopt msg))
                      ((= len 2) (loop (cddr args) (cons (cons arg (cadr args)) xs)))
                      (else (loop (cdr args) (cons (cons arg rest) xs))))))
            (else (error 'getopt msg))))))

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