anchor.diff:             collect (list variable (if (or (symbolp variable)
                                             (localp variable))
                                         (lower-env-form init-form)
  -                                      (list (sys.int::function-symbol '(setf %svref))
  +                                      (list (sys.int::function-symbol '(setf sys.int::%svref))
                                               (lower-env-form init-form)
                                               (second (first *environment-chain*))
                                               `',(1+ (position variable (gethash (first *environment*) *environment-layout*))))))))
  
positive.code:                                             :key #'third))
                         ,@(when (and (lambda-information-rest-arg lambda)
                                      (not (localp (lambda-information-rest-arg lambda))))
                                 (list (list (sys.int::function-symbol '(setf %svref))
                                             (lambda-information-rest-arg lambda)
                                             new-env
                                             `',(1+ (position (lambda-information-rest-arg lambda) local-env)))))
  
negative.code:                   (env-offset (1+ (position (second form) (gethash (first *environment*) *environment-layout*)))))
                (setf (block-information-env-var (second form)) env-var
                      (block-information-env-offset (second form)) env-offset)
                (list (list (sys.int::function-symbol '(setf sys.int::%svref))
                            (second form)
                            env-var
                            `',env-offset))))
  