title: How to increase value by a certain number?
body: <p>How to increase a value in a table by a certain number without reading last value and afterwards updating it?</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>product</th>
  <th>quantity</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>iLamp</td>
  <td>50</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I want to increase (or decrease) quantity by x. I am first reading last value (50), increasing or decreasing it, and writing it back. Is there a direct way?</p>