anchor.diff:       x))
   
   (defun expex-expr (ex expr)
  -  (let x (if (named-lambda? expr) ; XXX remove?
  -		     expr
  -			 (expex-guest-filter-expr ex expr))
  +  (let x (? (named-lambda? expr) ; XXX remove?
  +		    expr
  +		    (expex-guest-filter-expr ex expr))
       (expex-cps x)
  -    (if
  +    (?
         (%%vm-go-nil? x)
           (expex-vm-go-nil ex x)
   	  (%var? x)
  
positive.code: 
  (defun expex-vm-go-nil (ex x)
    (with ((moved new-expr) (expex-filter-and-move-args ex (list .x.)))
      (values (apply #'append moved)
              `((%%vm-go-nil ,@new-expr ,..x.)))))
  
  (defun peel-identity (x)
    (? (identity? x)
  
negative.code:   (funcall (expex-setter-filter ex) x))
  
  (defun expex-guest-filter-arguments (ex x)
    (mapcar (fn funcall (expex-argument-filter ex) _) x))
  
  ;;;; UTILS
  
  