switch (rep++) {
              case 0:
                return 0;
              case 1:
                return 1;
              case 2:
                return endOfData();
              default:
                throw new AssertionError("Should not have been invoked again");
            }
          }
        };

    assertTrue(iter.hasNext());

          return EndpointPair.ordered(requireNonNull(node), successorIterator.next());
        }
        if (!advance()) {
          return endOfData();
        }
      }
    }
  }

  /**
   * If the graph is undirected, each unordered [node, otherNode] pair (except self-loops) will be

    READY,
    NOT_READY,
    DONE,
    FAILED,
  }

  private @Nullable T next;

  protected abstract @Nullable T computeNext();

  @CanIgnoreReturnValue
  protected final @Nullable T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;

    READY,
    NOT_READY,
    DONE,
    FAILED,
  }

  private @Nullable T next;

  protected abstract @Nullable T computeNext();

  @CanIgnoreReturnValue
  protected final @Nullable T endOfData() {
    state = State.DONE;
    return null;
  }

  @Override
  public final boolean hasNext() {
    checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;

          T child = top.childIterator.next();
          stack.addLast(expand(child));
        } else {
          stack.removeLast();
          return top.root;
        }
      }
      return endOfData();
    }

    private PostOrderNode<T> expand(T t) {
      return new PostOrderNode<>(t, children(t).iterator());
    }
  }

  /**

          T child = top.childIterator.next();
          stack.addLast(expand(child));
        } else {
          stack.removeLast();
          return top.root;
        }
      }
      return endOfData();
    }

    private PostOrderNode<T> expand(T t) {
      return new PostOrderNode<>(t, children(t).iterator());
    }
  }

  /**