anchor.diff:      연산자(operator)와 익스텐션(extension)을 통해 지원을 확대할 수 있습니다.
       어떻게 하는지 곧 살펴보겠습니다.
   
  -    Angular core has basic support for observables.
  +    Angular core has basic support for observables. 
       We developers augment that support with operators and extensions from the
       <a href="http://reactivex.io/rxjs" target="_blank" title="RxJS">RxJS library</a>.
       We'll see how shortly.
  
positive.code:   여전히 !{_Promise}를 반환합니다.
    `getHeroes()`를 호출하는 어떤 컴포넌트도 수정하지 않아도 됩니다.
  
    Although we made significant *internal* changes to `getHeroes()` and `getHero()`,
    the public signatures did not change.
    We still return a !{_Promise} from both methods. 
    We won't have to update any of the components that call them.
  
negative.code:     각각의 경우, 라이브러리를 임포트하는 단순한 작업만으로
      라이브러리의 스크립트 파일을 로드하고 실행하며,
      결과적으로 `Observable` 클래스에 연산자를 추가합니다.
      
      We don't need the operator symbols themselves. 
      In each case, the mere act of importing the library
      loads and executes the library's script file which, in turn, adds the operator to the `Observable` class.
  
  