title: onEdit Event object access to previous value of a cell that has been modified?
body: <p>I cannot find any detailled documentation on the event.source object so I would like to know if it's possible to access the <em>previous</em> value of the modified range so I can compare the new values of the range with the old values of the range for validation.</p>
  
  <p>Thanks for your help.</p>
  
  <p>OnEdit(event)</p>
  
  <pre><code>function onEdit(event) 
  /* Default onEdit Event function */
  {
    var ssa = SpreadsheetApp.getActiveSpreadsheet();
    var ss = event.source.getActiveSheet();
    var r = event.source.getActiveRange();
  ...
  </code></pre>