public void ClearRedoCategory( 
   string category
)

{
  // in order to find a dockpane you need to know its DAML id
  pane = FrameworkApplication.DockPaneManager.Find(dockPaneID);

  // get the undo stack
  OperationManager manager = pane.OperationManager;
  if (manager != null)
  {
    // undo an operation
    // Use await with UndoAsync and RedoAsync
    if (manager.CanUndo)
      manager.UndoAsync().Wait();

    // redo an operation
    if (manager.CanRedo)
      manager.RedoAsync().Wait();

    // clear the undo and redo stack of operations of a particular category
    manager.ClearUndoCategory("Some category");
    manager.ClearRedoCategory("Some category");
  }
}