anchor.diff:                    (λ
                      (let (args)
                        (iterate (for input-iterator in-vector input-iterators downto 0)
  -                              (setf args (hcons (funcall input-iterator) args)))
  +                              (setf args (cons (funcall input-iterator) args)))
                        (%call (operator node) args)))))
                  ;; eliminate reference nodes entirely
                  (reference
  
positive.code: (defvar *recipe-space* nil)
  
  (defun map-recipes (function data-structure &key (leaf-test #'immediate?))
    "Apply FUNCTION to all recipes that compute values of DATA-STRUCTURE
     and reference data-structures that satisfy LEAF-TEST.
  
     For every recipe, FUNCTION receives the following arguments:
     1. the recipe
  
negative.code:     (let ((recipe-iterator
              (mkiter data-structure
                      (index-space data-structure)
                      (zero-based-transformation (index-space data-structure))
                      (dimension data-structure))))
        (handler-case
            (loop
  