anchor.diff:  (defstruct clutter-function operator)
   (defun make-function (operator)
     (make-clutter-function :operator operator))
  +
  +(defstruct clutter-symbol-operator operator)
  +(defun make-symbol-operator (operator)
  +  (make-clutter-symbol-operator :operator operator))
   
   (defun get-current-env () *denv*)
   
  
positive.code:                        (clutter-eval if-true *denv*)
                         (clutter-eval if-false *denv*))))))
  
  
  (defun clutter-true-p (exp)
    (if exp t nil))
  
negative.code:         (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)))
  
  (defun eval/combiner (expression env)
    (let ((f (clutter-eval (car expression) env)))
  