(define (copy-chars! n in out)
  (let loop ((n n))
    (or (= n 0)
        (let ((p (peek-char in)))
          (and (not (eof-object? p))
               (let* ((p2 (peek-char in))
                      (c  (read-char in)))
                 (unless (and (eqv? p2 p) (eqv? c p))
                   (error "peek-char/read-char disagree"
                          chars-read p p2 c))
                 (write-char c out)
                 (set! chars-read (+ chars-read 1))
                 (loop (- n 1))))))))