anchor.diff:    (without-interrupts
       (let* ((frame *snapshot-pending-writeback-pages*)
              (address (physical-page-virtual-address frame))
  -           (block-info (or (block-info-for-virtual-address address)
  -                           (panic "No block info for CoW address?" address)))
              (block-id (physical-page-frame-block-id frame)))
         ;; Remove frame from list.
         (setf *snapshot-pending-writeback-pages* (physical-page-frame-next frame))
  
positive.code:                  nil
                   block-id
                   address))
          (t (panic "Page " frame " for address " address " has non-writeback type "
                    (physical-page-frame-type frame)))))))
  
  (defun snapshot-write-back-pages ()
  
negative.code:          ;; Allow access to the page again.
           (let* ((pte (or (get-pte-for-address address nil)
                           (panic "No PTE for CoW address?")))
                  (frame (ash (pte-physical-address (sys.int::memref-unsigned-byte-64 pte 0)) -12))
                  ;; The block map should only be consulted for active pages.
                  ;; Other kinds of pages indicate that the virtual memory was modified
                  ;; and the page may no longer exist in the most recent block map.
                  (block-info (or (block-info-for-virtual-address address)
                                  (panic "No block info for CoW address?" address))))
             ;; Update PTE bits. Clear CoW bit, make writable.
             (setf (sys.int::memref-unsigned-byte-64 pte 0)
                   (make-pte frame
  