title: DataGrid - collapse all groups except the first one
body: <p>I have a DataGrid with grouped ItemsSource. There are an expander on each group, so I can expand/collapse all the groups. Now, I'm trying to collapse all groups by default, but leave the first group expanded. The items source is dynamic, so I can't build any converter to check the group name. I must do it by group index.</p>
  
  <p>Is it possible to do in in XAML? Or in code-behind?</p>
  
  <p>Please, help.</p>