code: def get_main_chain_layers(self):
          
          main_chain = self.get_main_chain()
          ret = []
          for u in main_chain:
              for v, layer_id in self.adj_list[u]:
                  if v in main_chain and u in main_chain:
                      ret.append(layer_id)
          return ret
docs: Return a list of layer IDs in the main chain.