anchor.diff:  ;; last expression assigned to a gensym which will replace
   ;; it in the parent expression.
   (defun expex-body (ex x &optional (s '~%ret))
  -  (when (not x)	; Encapsulate NIL.
  +  (unless x	; Encapsulate NIL.
   	(setf x '((identity nil))))
     (with (e (expex-list ex x))
      	(expex-make-return-value ex e s)))
  
positive.code:     (values (apply #'append pre)
  			main)))
  
  (defun expex-argexpand-do (ex fun args)
    (funcall (expex-function-collector ex) fun args)
    (if (funcall (expex-plain-arg-fun? ex) fun)
  	  args
        (argument-expand-compiled-values fun
  								       (funcall (expex-function-arguments ex)
  												fun)
  								       args)))
  
  (defun expex-argexpand (ex fun args)
    (if (and (atom fun)
  
negative.code: ;; Recurses into LAMBDA-expressions and VM-SCOPEs.
  ;; Removes VM-SCOPEs.
  (defun expex-expr (ex x)
    (if
  	(lambda? x)
        (values nil (list `#'(lambda ,(lambda-args x)
  						     ,@(expex-body ex (lambda-body x)))))
      (not (expex-able? ex x))
        (values nil (list x))
      (vm-scope? x)
  	  (values nil (expex-body ex (vm-scope-body x)))
      (expex-std-expr ex x)))
  
  ;; Entry point.
  ;;
  