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