Observadores

• 

  Karl Hoag

  https://ruuby.us/%EB%A1%9C%EB%98%90%ED%8C%90%EB%A7%A4%EC%A0%90-%ED%96%89%EC%9A%B4%EC%9D%84-%EC%A3%BC%EB%8A%94-%EC%9E%A5%EC%86%8C/