anchor.diff:                         (make-%= '~%ret _)])
       (expex-make-return-value s 
           (mapcan [with ((moved new-expr) (expex-expr _))
  -                 (append moved (mapcan #'ensure-%= new-expr))]
  +                  (+ moved (mapcan #'ensure-%= new-expr))]
                   (wrap-atoms (list-without-noargs-tag x))))))
   
   
  
positive.code: 
  (defvar *expex* nil)
  
  (define-gensym-generator expex-sym e)
  
  
  ;;;; GUEST CALLBACKS
  
negative.code:   (expex-guest-filter-setter `(%= ,p ,(? (%=? v)
                                           (%=-place v)
                                           v))))
  
  (define-gensym-generator expex-sym e)
  
  (defun expex-add-var ()
    (funinfo-var-add *funinfo* (expex-sym)))
  