anchor.diff:  
   (register-descriptor-and-constructors :base16 (base16-format-descriptor)
                                         #'make-base16-encode-state
  -                                      #'make-base16-encode-state)
  +                                      #'make-base16-decode-state)
   (register-descriptor-and-constructors :hex (base16-format-descriptor)
                                         #'make-hex-encode-state
  -                                      #'make-hex-encode-state)
  +                                      #'make-hex-decode-state)
  
positive.code:   (base16-encoder state output input output-start output-end
                    input-start input-end lastp #'identity))
  
  (defun string->octets/base16 ()
    )
  
  (defun octets->octets/decode/base16 ()
    )
  
  (defun decode-octets-base16 (string start end length table writer)
    (declare (type index start end))
  
negative.code:                      (error "Invalid hex digit ~A" char1))
                     (when (= v2 +dt-invalid+)
                       (error "Invalid hex digit ~A" char2))
                     (funcall writer (+ (* (logand v1 #xf) 16)
                                        (logand v2 #xf)))))))
      (declare (inline do-decode))
      (decode-dispatch string #'do-decode)))
  
  