anchor.diff:    (with-open-file (f path
                        :direction :input
                        :element-type '(unsigned-byte 8))
  -    (when (= (file-position f offset) offset)
  +    (when (file-position f offset)
         (let ((bytes (make-array *fingerprint-length* :element-type '(unsigned-byte 8))))
           (read-sequence bytes f)
           (every #'= bytes *fingerprint*)))))
  
positive.code:   (with-open-file (f path
                       :direction :input
                       :element-type '(unsigned-byte 8))
      (when (= (file-position f offset) offset)
        (let ((token (make-array *token-length* :element-type '(unsigned-byte 8))))
          (read-sequence token f)
          token))))
  
negative.code:                      :direction :output
                       :if-exists :overwrite
                       :element-type '(unsigned-byte 8))
      (when (file-position f offset)
        (write-sequence token f))))
  