anchor.diff:            (apply #'= (mapcar #'scalar-magnitude 
                                (list m m-2 m-3)))))
   ;; => #<METER 1 m {1006330FD3}>, 
  -;;    #<METER (1/1000)E+3 km {1006331AF3}>, 
  -;;    #<METER 1000E-3 mm {1006332293}>, 
  +;;    #<METER 1 m {1006331AF3}>, 
  +;;    #<METER 1 m {1006332293}>, 
   ;;    T 
   ;; ^ Implementation note: The magnitude of the measurement is 1,
   ;;   in each of those three instances. 
  
positive.code:        (m-2 (nrescale m 3)))
    (values m-2 (eq m-2 m)))
  
  ;; => #<METER 0.001E+3 km {1005F4C4D3}>, 
  ;;    T
  
  
  
negative.code:              (boundp
                (multiple-value-bind (adj-mag deg)
                    (scale-for-si-degree mag deg)
                    (princ adj-mag stream)
                    (write-char #\Space stream)
                  (unless (zerop deg)
                    (let ((prefix (find-prefix= deg))) 
                      (princ (prefix-print-name prefix) stream)))))
               (t (princ mag stream)))))
          (t
           (princ "{no magnitude}" stream))))
  