anchor.diff:  
   (defconstant +ensure-class-origin+ #'c2mop:ensure-class)
   
  -(defun start-metap-ensure-class ()
  +(defun enable-metap ()
     #+sbcl
     (sb-ext:without-package-locks
       (fmakunbound 'c2mop:ensure-class)
  
positive.code:   (:export :*metap-m1-m2-pairs*
             :register-m1-m2-pair
             :validate-superclass*
             :with-metap-ensured))
  (in-package :metap)
  
  
negative.code:     `(let ((,origin #'c2mop:ensure-class))
         (unwind-protect
              (progn
                (enable-metap)
                (progn ,@body))
           (disable-metap ,origin)))))
  