anchor.diff:                      (with block := (read-block disk superblock 1
                                                  (ceiling (/ n-octets
                                                              (block-device-sector-size disk)
  -                                                           (block-size disk superblock)))))
  +                                                           (sectors-per-block disk superblock)))))
                       (for offset :from 0 :below n-octets :by block-group-size)
                       (collecting (read-block-group-descriptor superblock block offset)))))
   
  
positive.code:   (let ((host (host stream)))
      (read-block-n (file-host-mount-device host)
                    (superblock host)
                    (bgdt host)
                    (file-inode stream)
                    block-n)))
  
  
negative.code:         :with file-name := (file-name pathname)
          :for directory :in (rest (pathname-directory pathname))
          :do (block do-files
                (do-files (block offset) disk superblock inode
                  (error 'simple-file-error
                         :pathname pathname
                         :format-control "Directory ~A not found. ~S"
  