anchor.diff:                    (make-symbol-operator (car values))))))
   
   (defun clutter-true-p (exp)
  -  (if exp t nil))
  +  (if (not (eq exp :false)) t nil))
  
positive.code:      (make-clutter-operator
        :function (lambda (*denv* values)
                    (cadr values)))))
  (defprimitive list (make-function
                      (make-clutter-operator
                       :function (lambda (*denv* values)
  
negative.code:         (t expression)))
  
  (defun eval/symbol (symbol env)
    (if (keywordp symbol)
        symbol
        (let ((val (lookup symbol env)))
          (if (clutter-symbol-operator-p val)
              (invoke (clutter-symbol-operator-operator val) env nil)
              val))))
  
  (defun eval/combiner (expression env)
    (let ((f (clutter-eval (car expression) env)))
  