anchor.diff:  	 (error "generic function class must be a class or a name")))
     (when method-class-p
       (cond ((symbolp method-class)
  -	   (let ((class (find-class method-class nil)))
  +	   (let ((class (sicl-environment:find-class method-class
  +						     environment)))
   	     (when (null class)
   	       (error "no such method class ~s" method-class))
   	     (setf method-class class)))
  
positive.code:      &rest
         all-keyword-arguments
       &key
         (generic-function-class (find-class 'standard-generic-function))
         (method-class nil method-class-p)
       &allow-other-keys)
    (declare (ignore generic-function))
    (cond ((symbolp generic-function-class)
  	 (let ((class (find-class generic-function-class nil)))
  	   (when (null class)
  	     (error "no such generic-function-class ~s"
  		    generic-function-class))
  
negative.code:     (error "classes don't agree"))
    (when method-class-p
      (cond ((symbolp method-class)
  	   (let ((class (sicl-environment:find-class method-class
  						     environment)))
  	     (when (null class)
  	       (error "no such method class ~s" method-class))
  	     (setf method-class class)))
  