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