anchor.diff:  
   block observables-section-intro
     :marked
  -    Each `Http` method  returns an `Observable` of HTTP `Response` objects.
  +    Each `Http` service method  returns an `Observable` of HTTP `Response` objects.
   
       Our `HeroService` converts that `Observable` into a `Promise` and returns the promise to the caller.
       In this section we learn to return the `Observable` directly and discuss when and why that might be
  
positive.code: 
    .l-sub-section
      :marked
        The [switchMap operator](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/flatmaplatest.md)
        (formerly known as "flatMapLatest") is very clever.
  
        Every qualifying key event can trigger an http call.
        Even with a 300ms pause between requests, we could have multiple http requests in flight
        and they may not return in the order sent.
  
        `switchMap` preserves the original request order while returning
         only the observable from the most recent http call.
        Results from prior calls are canceled and discarded.
  
        We also short-circuit the http call and return an observable containing an empty array
        if the search text is empty.
  
        Note that _canceling_ the `HeroSearchService` observable won't actually abort a pending http request
        until the service supports that feature, a topic for another day.
        We are content for now to discard unwanted results.
    :marked
  
negative.code: h1 Providing HTTP Services
  block http-library
    :marked
      The `HttpModule` is ***not*** a core Angular module.
      It's Angular's optional approach to web access and it exists as a separate add-on module called `@angular/http`,
      shipped in a separate script file as part of the Angular npm package.
  
      Fortunately we're ready to import from `@angular/http` because `systemjs.config` configured *SystemJS* to load that library when we need it.
  
  :marked
    ### Register for HTTP services
  
  block http-providers
    :marked
      Our app will depend upon the Angular `http` service which itself depends upon other supporting services.
      The `HttpModule` from `@angular/http` library holds providers for a complete set of HTTP services.
  
      We should be able to access these services from anywhere in the application.
      So we register them all by adding `HttpModule` to the `imports` list of the `AppModule` where we
      bootstrap the application and its root `AppComponent`.
  
    +makeExample('app/app.module.ts', 'v1','app/app.module.ts (v1)')
  