anchor.diff:  
      
   
  -
  -#+SBCL
  +#+SBCL ;; FIXME: also port to other impmls. using PCL
   (defmethod sb-mop:validate-superclass ((class measurement-class)
                                          (superclass standard-class))
     (values t))
  
positive.code: 
  
  
  ;; FIXME: does not work in SBCL, but should.
  ;;
  ;; In macroexapansion for DEFBOUNCE%, NAME is said to be unbound
  ;;
  
negative.code: 
  (let ((%classes% (make-array 7 :fill-pointer 0))
        (%classes-lock% (make-lock "%CLASSES%")))
    (defun register-measurement-class (c)
      (with-lock-held (%classes-lock%)
        (let* ((s (measurement-symbol c))
  