anchor.diff:  :marked
     ### NgFor
   
  -  ### NgFor
  -
     `NgFor` is a _repeater_ directive &mdash; a way to customize data display.
   
     `NgFor`是一个_重复器_指令 —— 自定义数据显示的一种方式。
  
positive.code: :marked
    ### NgIf
  
    ### NgIf
  
    We can add an element subtree (an element and its children) to the DOM  by binding an `NgIf` directive to a #{_truthy} expression.
  
    通过绑定`NgIf`指令到真值表达式，可以把元素子树（元素及其子元素）添加到 DOM 上。
  
negative.code: :marked
    ### NgSwitch
  
    We bind to `NgSwitch` when we want to display *one* element tree (an element and its children)
    from a *set* of possible element trees, based on some condition.
    Angular puts only the *selected* element tree into the DOM.
  