anchor.diff:  
   
     <tr>
  +    <%= form_for :category, url: {action: 'create'} do |f| %>
       <td>
         &nbsp;
       </td>
  
positive.code:     <td>
        <%= f.submit('+') %>
      </td>
    </tr>
    </tbody>
  
  
  </table>
  
  <% end %>
  
  
  
  
negative.code: <h1>Categorie di libri</h1>
  
  <table border="1">
  
    <thead>
  