* info.
   *
   * @throws IllegalArgumentException if {@code startNode} is not an element of the graph
   */
  public final Iterable<N> breadthFirst(N startNode) {
    return breadthFirst(ImmutableSet.of(startNode));
  }

  /**
   * Returns an unmodifiable {@code Iterable} over the nodes reachable from any of the {@code
   * startNodes}, in the order of a breadth-first traversal. This is equivalent to a breadth-first