anchor.diff:    the component's behavior continues.
     It remains attached to its DOM element. It continues to listen to events.
     Angular keeps checking for changes that could affect data bindings.
  -  Whatever the component was doing it keeps doing.
  -
  -  Although invisible, the component &mdash; and all of its descendent components &mdash;
  +  Whatever the component was doing, it keeps doing it.
  +
  +  Although invisible, the component &mdash; and all of its descendant components &mdash;
     tie up resources that might be more useful elsewhere.
     The performance and memory burden can be substantial and the user may not benefit at all.
   
  
positive.code:   a heavy price to create and destroy it.
  
    If we really expected to "wink" the component like this, toggling visibility would be the better choice.
    In most UIs, when we "close" a component we're unlikely see it again for a long time, if ever.
    The `ngIf` would be preferred in that case.
  
  <a id="template"></a>
  
negative.code: :marked
    ## NgIf Case Study
  
    Let's focus on `ngIf`. It's a great example of a structural
    directive: it takes a boolean and makes an entire chunk of DOM appear
    or disappear.
  
  