title: How to "cancel" a UIStoryBoardSegue
body: <p>Does anyone know how to "stop" a segue transition conditionally:</p>
  
  <p>My table view cells represent products which can be viewed in a drill-down "detail" view... or  cannot! (It depends on a couple of things)</p>
  
  <p>Now my App considers all products "unlocked":</p>
  
  <pre><code>- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
  {
      NSIndexPath *selectedRowIndex = [self.tableView indexPathForSelectedRow];
      ListaProdottiController *prodottiViewController = [segue destinationViewController];
      prodottiViewController.blocco = [self.fetchedResultsController objectAtIndexPath:selectedRowIndex];
  }
  </code></pre>
  
  <p>How can I cancel the row selection => drilldown,  at this point?</p>