anchor.diff:    </div>
     <div class="form-group">
     <%= f.submit "Sign in", class: "btn btn-default btn-primart" %>
  +
     </div>
     <% end %>
   </div>
  
positive.code: <% provide(:title, "Sign in") %>
  <h2>Sign in</h2>
  <div class="container">
       <div class="col-md-4">
       </div>
  
negative.code:   <div class="form-group">
    <%= form_for(:session, url: session_path) do |f| %>
    <div class="form-group">
    <%= f.label :name, "用户名", class: "control-label col-sm-1" %>
    <%= f.text_field :name, class: "form-control" %>
    </div>
    <div class="form-group">
  