anchor.diff:          (t expression)))
   
   (defun eval/symbol (symbol env)
  -  (let ((val (lookup symbol env)))
  -    (if (clutter-symbol-operator-p val)
  -        (invoke (clutter-symbol-operator-operator val) env nil)
  -        val)))
  +  (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)))
  
positive.code:                   (make-symbol-operator (car values))))))
  
  (defun clutter-true-p (exp)
    (if exp t nil))
  
negative.code:      (make-clutter-operator
        :function (lambda (*denv* values)
                    (cadr values)))))
  
  (defprimitive list (make-function
                      (make-clutter-operator
                       :function (lambda (*denv* values)
  