anchor.diff:  
   (register-descriptor-and-constructors :base64 (base64-format-descriptor)
                                         #'make-base64-encode-state
  -                                      #'make-base64-encode-state)
  +                                      #'make-base64-decode-state)
   (register-descriptor-and-constructors :base64url (base64-format-descriptor)
                                         #'make-base64url-encode-state
  -                                      #'make-base64url-encode-state)
  +                                      #'make-base64url-decode-state)
  
positive.code: (defvar *base64url-decode-table*
    (make-decode-table *base64url-encode-table*))
  (declaim (type decode-table *base64url-decode-table*))
  
  (defun decode-octets-base64 (string start end length table writer)
    (declare (type index start end))
  
negative.code:   (base64-encoder state output input output-start output-end
                    input-start input-end lastp #'identity))
  
  (defun encoded-length/base64 (count)
    "Return the number of characters required to encode COUNT octets in Base64."
    (* (ceiling count 3) 4))
  