Observadores

• 

  Henry Venn

  https://yiilu.net/%EB%A8%B9%ED%8A%80%EA%B2%80%EC%A6%9D%EC%9D%98-%EC%A4%91%EC%9A%94%EC%84%B1%EA%B3%BC-%EC%8A%88%EC%96%B4%EB%A7%A8-%EC%BB%A4%EB%AE%A4%EB%8B%88%ED%8B%B0%EC%9D%98-%EC%97%AD%ED%95%A0/