anchor.diff:  
     (defun find-measurement-class (s)
       (declare (type symbol s)
  -             (values measurement-class))
  +             (values measurement-class &optional))
       (with-lock-held (%classes-lock%)
         (or (find s %classes%
  -               :test #'eq
  -               :key #'measurement-symbol)
  +                :test #'eq
  +                :key #'measurement-symbol)
             (error 'class-not-found :name s))))
     )
     
  
positive.code:   (values t))
  
  
  (let ((%classes% (make-array 7 :fill-pointer 0))
        (%classes-lock% (make-lock "%CLASSES%")))
    (defun register-measurement-class (c)
      (declare (type measurement-class c))
  
negative.code:            (values real))
    (let ((deg (measurement-degree m)))
      (cond 
        ((zerop deg) (values (measurement-magnitude m)))
        (t (values (* (measurement-magnitude m)
                      (expt #.%factor-base% deg)))))))
  
  ;; (base-magnitude (make-measurement 1 :m 3))
  ;; => 1000
  