anchor.diff:                       (cons (pair (car x) (car y))
                              (zip (cdr x) (cdr y)))))))
   
  -(label assoc (lambda (x y)
  +(label lookup (lambda (x y)
                  (cond ((eq (car (car y)) x) (car (cdr (car y))))
  -                     (t (assoc x (cdr y))))))
  +                     (t (lookup x (cdr y))))))
   
   (label caar (lambda (x) (car (car x))))
   (label cadr (lambda (x) (car (cdr x))))
  
positive.code: 
  (label eval (lambda (expr binds)
                (cond
                  ((atom expr) (assoc expr binds))
                  ((atom (car expr))
                   (cond
                     ((eq (car expr) (quote quote)) (cadr expr))
  
negative.code:                    ((eq (car expr) (quote cons))  (cons   (eval (cadr expr) binds)
                                                            (eval (caddr expr) binds)))
                     ((eq (car expr) (quote cond))  (eval-cond (cdr expr) binds))
                     (t (eval (cons (lookup (car expr) binds)
                                    (cdr expr))
                              binds))))
                  ((eq (caar expr) (quote label))
  