anchor.diff:  	(make-measurement m class d)))))
   
   ;; 5*10^2 + 5*10^2  = 10^2(5 + 5)) = 1000
  -;; (scalar-magnitude (@+ (make-measurement 5 :m 2) (make-measurement 5 :m 2)))
  +;; (scalar-magnitude (@+ (make-measurement 5 :|m| 2) (make-measurement 5 :|m| 2)))
   ;; => 1000
   
   ;; 5*10^2 + 5*10^3  = 10^2(5*10^0 + 5*10^1) = 5500
  -;; (scalar-magnitude (@+ (make-measurement 5 :m 2) (make-measurement 5 :m 3)))
  +;; (scalar-magnitude (@+ (make-measurement 5 :|m| 2) (make-measurement 5 :|m| 3)))
   ;; => 5500
  -;; (scalar-magnitude (@+ (make-measurement 5 :m 3) (make-measurement 5 :m 2)))
  +;; (scalar-magnitude (@+ (make-measurement 5 :|m| 3) (make-measurement 5 :|m| 2)))
   ;; => 5500
   
   ;; 5*10^-2 + 5*10^3 = 10^-2(5*10^0 + 5*10^5) = 100001/20 = 5000.05
  -;; (scalar-magnitude (@+ (make-measurement 5 :m -2) (make-measurement 5 :m 3)))
  +;; (scalar-magnitude (@+ (make-measurement 5 :|m| -2) (make-measurement 5 :|m| 3)))
   ;; => 100001/20
  -;; (scalar-magnitude (@+ (make-measurement 5 :m 3) (make-measurement 5 :m -2)))
  +;; (scalar-magnitude (@+ (make-measurement 5 :|m| 3) (make-measurement 5 :|m| -2)))
   ;; => 100001/20
   
   (defmethod %- ((a measurement))
  
positive.code: 	(make-measurement m class d)))))
  
  ;; 5*10^2 - 5*10^2  = 10^2(5 - 5)) = 0
  ;; (scalar-magnitude (@- (make-measurement 5 :m 2) (make-measurement 5 :m 2)))
  ;; => 0
  
  ;; 5*10^2 - 5*10^3  = 10^2(5*10^0 - 5*10^1) = -4500
  ;; (scalar-magnitude (@- (make-measurement 5 :m 2) (make-measurement 5 :m 3)))
  ;; => -4500
  
  ;; 5*10^-2 - 5*10^3 = 10^-2(5*10^0 - 5*10^5) = -99999/20 = -4999.95
  ;; (scalar-magnitude (@- (make-measurement 5 :m -2) (make-measurement 5 :m 3)))
  ;; => -99999/20
  
  
  (defmethod %* ((a measurement))
    (values a)))
  
  #+NIL ;; FIXME: Must return measurement of type unit^2
  (defmethod @* ((a measurement) (b measurement))
  
negative.code: 		     (class-of a)
  		     (measurement-degree a))))
  
  ;; FIXME: Must return measurement of a "dimensionless" unit
  #+NIL
  (defmethod @/ ((a measurement) (b measurement))
    (ERROR "Frob"))
  