anchor.diff:    (cond
       [(> i (n li)) false]
       [(fn (nth li i)) true]
  -    [true (fast-any fn li (+ i 1))]))
  +    [:else (fast-any fn li (+ i 1))]))
  
positive.code:   (cond
      [(> i (n li)) true]
      [(fn (nth li i)) (fast-all fn li (+ i 1))]
      [true false]))
  
  (defun fast-any (fn li i)
    "A fast implementation of any which starts from an offset.
  
negative.code:        [(= (.> builtins :true)  (.> node :var)) true]
         [(= (.> builtins :false) (.> node :var)) false]
         [(= (.> builtins :nil)   (.> node :var)) false]
         [:else nil])]
      [:else nil]))
  
  (defun make-progn (body)
    "Allow using BODY as an expression."
  