anchor.diff:  :marked
     ## NgIf Case Study
   
  -  Let’s focus on `ngIf`. It's a great example of a structural
  +  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.
   
  
positive.code:   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;
    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.
  
  
negative.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 to see it again for a long time, if ever.
    The `ngIf` would be preferred in that case.
  
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