if (successors.hasNext()) {
                // BFS: horizon.addLast(successors)
                // Pre-order: horizon.addFirst(successors)
                order.insertInto(horizon, successors);
              }
              return next;
            }
          } while (!horizon.isEmpty());
          return endOfData();
        }
      };
    }

// https://www.iana.org/domains/root/db/xn--1qqw23a.html
佛山

// xn--30rr7y : Excellent First Limited
// https://www.iana.org/domains/root/db/xn--30rr7y.html
慈善

// xn--3bst00m : Eagle Horizon Limited
// https://www.iana.org/domains/root/db/xn--3bst00m.html
集团

// xn--3ds443g : TLD REGISTRY LIMITED OY
// https://www.iana.org/domains/root/db/xn--3ds443g.html
在线