anchor.diff:    (function-collector #'((fun args)))
   
     ; Callback to collect used variables.
  -  (argument-filter #'((var)))
  +  (argument-filter #'((var) var))
  +
  +  (setter-filter #'((var) var))
   
     (plain-arg-fun? #'((var)))
   
  
positive.code: (defun expex-expr-setq (ex x)
    (with ((moved new-expr) (expex-args ex (cddr x)))
  	(values moved
  		    `((%setq ,(second x) ,@new-expr)))))
  
  (defun expex-lambda (ex x)
    (with-temporary *expex-funinfo* (get-lambda-funinfo x)
  
negative.code:   (and (atom x)
         (string= "~E" (subseq (symbol-name x) 0 2))))
  
  (defun expex-filter-setter (ex x)
    (funcall (expex-setter-filter ex) x))
  
  (defun expex-filter-arguments (ex x)
    (mapcar (fn (funcall (expex-argument-filter ex) _))
  		  x))
  