anchor.diff:  
     `ContactComponent`的选择器会去匹配名叫`<app-contact>`的元素。
     在`AppComponent`模板中`<app-title>`的下方添加一个具有此名字的元素：
  +  
   +makeExample('ngmodule/ts/src/app/app.component.1b.ts', 'template', 'src/app/app.component.ts (template)')(format=".")
   
   :marked
  
positive.code:     我们只要在 import 时使用`as`关键字来为第二个指令创建个别名就可以了。
      
    +makeExample('ngmodule/ts/src/app/app.module.1b.ts', 'import-alias')(format=".")
    :marked
      This solves the immediate issue of referencing both directive _types_ in the same file but
      leaves another issue unresolved.
  
negative.code:   注意，在修改过的`TitleComponent`中，有一个`*ngIf`指令在“守卫着”该消息。如果没有当前用户，就没有任何消息。
  
  +makeExample('ngmodule/ts/src/app/title.component.html', 'ngIf', 'src/app/title.component.html (ngIf)')(format=".")
  
  :marked
    Although `AppModule` doesn't declare `NgIf`, the application still compiles and runs.
    How can that be? The Angular compiler should either ignore or complain about unrecognized HTML.
  