anchor.diff:    We know the only `LoggerService` instance in the app is provided at the `AppComponent` level.
     The host `HeroBioComponent` doesn't have its own `LoggerService` provider.
     
  -  另外一个`@Host()`函数装饰`_loggerService`属性，我们知道在应用程序中，只有一个`LoggerService`实例，在`AppComponent`级别提供的。该主持`HeroBioComponent`没有自己的`LoggerService`provider。
  +  另外一个`@Host()`函数装饰`_loggerService`属性，我们知道在应用程序中，只有一个`LoggerService`实例，在`AppComponent`级别提供的。该主持`HeroBioComponent`没有自己的`LoggerService`供应商。
     
     Angular would throw an error if we hadn't also decorated the property with the `@Optional()` function.
     Thanks to `@Optional()`, Angular sets the `loggerService` to null and the rest of the component adapts.
  
positive.code:   that creates an *alternative* way to inject the same component instance
    and add that provider to the `providers` array of the `@Component` metadata for the `AlexComponent`:
    
    我们编写一个[*别名provider*](#useexisting) &mdash；一个拥有`useExisting`定义的`provide`函数 &mdash;
    它新建一个*可选的*方法来注入一样的组件实例，并添加这个provider到`AlexComponent`的`@Component`元数据里的`providers`数组。
  a(id="alex-providers")
  +makeExample('cb-dependency-injection/ts/app/parent-finder.component.ts','alex-providers','parent-finder.component.ts (AlexComponent providers)')(format='.')
  :marked
    [Parent](#parent-token) is the provider's *class-interface* token. 
    The [*forwardRef*](#forwardref) breaks the circular reference we just created by having the `AlexComponent` refer to itself.
    
    [父级](#parent-token)是该provider的*类-接口*令牌。我们刚通过在`AlexComponent`引用了自己的建立了一个引用循环， 这个[*forwardRef*](#forwardRef)打破了该循环。
    *Carol*, the third of *Alex*'s child components, injects the parent into its `parent` parameter, the same way we've done it before:
    
    *Carol*，该*Alex*子级组件的第三个组件，注入了父级到自己的`parent`参数，和我们之前做的一样：
  
negative.code:   Here we provide the `HeroService` to the `HeroesBaseComponent` by listing it in the `providers` array:
    
    通过*在该分支的子级根部组件*中提供该服务， 我们能把一个注入服务的作用范围局限到一个应用程序结构的该*分支*中。
    这里我们通过列入`供应商`数列，在`HeroesBaseComponent`中提供了`HeroService`：
  +makeExample('cb-dependency-injection/ts/app/sorted-heroes.component.ts','injection','app/sorted-heroes.component.ts (HeroesBaseComponent excerpt)')
  :marked
    When Angular creates the `HeroesBaseComponent`, it also creates a new instance of `HeroService` 
  