anchor.diff:      <td>
         <%= f.submit('+') %>
       </td>
  +    <% end %>
     </tr>
     </tbody>
   
   
   </table>
   
  -<% end %>
   
   
   
  
positive.code: 
  
    <tr>
      <td>
        &nbsp;
      </td>
  
negative.code: <h1>Categorie di libri</h1>
  
  <table border="1">
  
    <thead>
  