anchor.diff:                                                  (plus x ypos)) do
                              (crunch sampled-buf pos q-tab)
                              (setf (aref preds cn)
  -                                 (encode-block (zigzag (aref sampled-buf pos))
  +                                 (encode-block (zigzag (aref sampled-buf pos) zz-result)
                                                  hufftabs (aref preds cn) out-stream)))))))))
       (unless (zerop *prev-length*)
         (write-stuffed (deposit-field #xff ; byte padding & flushing
  
positive.code:     (setq sampling '((1 1))))
    (let* ((wd (loop for entry in sampling maximize (the fixnum (first entry))))
           (ht (loop for entry in sampling maximize (the fixnum (second entry))))
  	 (*zz-result* (make-array 64 :element-type 'sint16))
  	 (*prev-byte* 0) ; State variables for write-bits
  	 (*prev-length* 0)
           (isampling (convert-sampling sampling wd ht))
  
negative.code: 
  ;;; Emits q-tables
  (defun write-quantization-tables (tables s)
    (let ((len (plus 2 (mul 65 (length tables))))
          (zz-result (make-array 64 :element-type 'uint8)))
      (write-marker +M_DQT+ s)
      (write-byte (ash len -8) s) ; MSB
      (write-byte (logand len #xff) s) ; LSB
      (loop for table across tables
            for i fixnum from 0 do
            (write-byte i s)
            (write-sequence (zigzag8 table zz-result) s))))
  
  ;;; Emits huffman tables in the following order:
  ;;; luminance DC
  