agenda는 시뮬레이션 시간을 명시적인 데이터로 만든다.

(begin
  (define (schedule time action agenda)
    (let ((event (list time action)))
      (cond ((null? agenda) (list event))
            ((< time (car (car agenda))) (cons event agenda))
            (else
             (cons (car agenda)
                   (schedule time action (cdr agenda)))))))
  (define agenda
    (schedule 5 'close
      (schedule 2 'sense
        (schedule 5 'open '()))))
  agenda)