anchor.diff:      ((cond-p exp) (t-eval (cond->if exp) env))
       ((and-p exp) (eval-and exp env))
       ((or-p exp) (eval-or exp env))
  +    ((let-p exp) (t-eval (let->combination exp) env))
       ((begin-p exp) (eval-sequence (begin-actions exp) env))
       ((application-p exp) (eval-application exp env))
       (t 'not-implemented)))
  
positive.code: 		       (iter (rest-exps exps)))))))
      (iter (and-exps exp))))
  
  ;; begin
  
  (defun begin-p (exp)
  
negative.code:       (iter (and-exps exp))))
  
  ;; or
  
  (defun or-p (exp)
    (tagged-list-p exp 'or))
  
  