anchor.diff:               (boundp
                 (multiple-value-bind (adj-mag deg)
                     (scale-for-si-degree mag deg)
  -                (let ((prefix (find-prefix= deg))) 
                     (princ adj-mag stream)
                     (write-char #\Space stream)
  -                  (princ (prefix-print-name prefix) stream))))
  +                (unless (zerop deg)
  +                  (let ((prefix (find-prefix= deg))) 
  +                    (princ (prefix-print-name prefix) stream)))))
                (t (princ mag stream)))))
           (t
            (princ "{no magnitude}" stream))))
  
positive.code:        (m-2 (nrescale m 3)))
    (values m-2 (eq m-2 m)))
  
  ;; => #<METER 0.001E+3 km {1005F4C4D3}>, 
  ;;    T
  
  
  
negative.code:           (apply #'= (mapcar #'scalar-magnitude 
                               (list m m-2 m-3)))))
  ;; => #<METER 1 m {1006330FD3}>, 
  ;;    #<METER 1 m {1006331AF3}>, 
  ;;    #<METER 1 m {1006332293}>, 
  ;;    T 
  ;; ^ Implementation note: The magnitude of the measurement is 1,
  ;;   in each of those three instances. 
  