anchor.diff:  
   :marked
     ### Expanding `*ngSwitch`
  +
     ### 展开`*ngSwitch`
     
     A similar transformation applies to `*ngSwitch`. We can unfold the syntax ourselves.
  
positive.code: :marked
    <a id="safe-navigation-operator"></a>
    ### The safe navigation operator ( ?. ) and null property paths
    ### 安全导航操作符( ?. )和空属性路径
  
    The Angular **safe navigation operator (`?.`)** is a fluent and convenient way to guard against null and undefined values in property paths.
  
negative.code: .l-main-section
  :marked
    ## Template reference variables
  
    ## 模板引用变量
  
    A **template reference variable** is a reference to a DOM element or directive within a template.
  