(define my_and
  (lambda (x y)
    (if x
	y
	#f)))

(define lazy_and
  (lambda (x y)
    (if (x)
	(y)
	#f)))

(define lazy-evaluation
  (lambda (t)
    (let ([val #f] [flag #f])
      (lambda ()
	(if (not flag)
	    (begin (set! val (t))
		   (set! flag #t)
		   val)
	    val)))))