code: def fn_x(i, dfs_data):
      
      try:
          return R(i, dfs_data)['x']
      except Exception as e:
          # Page 17 states that if Ri is empty, then we take xi to be n
          return dfs_data['graph'].num_nodes()
docs: The minimum vertex (DFS-number) in a frond contained in Ri.