code: def handle_selected_page(self):
          
          if not self.selected_page:
              pass
          if self.selected_page.name in ('subscription', 'submission'):
              # Launch page in a subwindow
              self.selected_page = self.selected_page.loop()
          elif self.selected_page.name in ('subreddit', 'inbox'):
              # Replace the current page
              self.active = False
          else:
              raise RuntimeError(self.selected_page.name)
docs: Open the subscription and submission pages subwindows, but close the
          current page if any other type of page is selected.