anchor.diff:                  (invoke func (mapcar #'funcall pretreated-args))
                   (error "Not a function: " (car expression))))))))
   
  +(defun pretreat/symbol (expression env)
  +  (lambda ()
  +    (lookup expression :lexical)))
  +
   (defun pretreat/function-ref (name env)
     (lambda ()
       (lookup name :function)))
  
positive.code:     (pretreat/function-ref var env)))
  
  (defun pretreat/bind-lexical-variables (expression env)
    ;; TODO
    )
  
  (defun pretreat/set-lexical-variables (expression env)
    ;; TODO
    )
  
  (defun pretreat/var (expression env)
    ;; TODO
  
negative.code:   (assert (clutter-symbol-p operator))
    (or (cdr (assoc (clutter-symbol-name operator) *pretreaters* :test #'equal))
        #'pretreat/application))
  
  (defun pretreat (expression env)
    (if (atom expression)
  