@Override
	public Stream<FlatTreeNode<V>> breadthFirstStream() {
		return isRoot()
			? IntStream.range(0, _nodes.values.length).mapToObj(this::nodeAt)
			: FlatTree.super.breadthFirstStream();
	}
Return the index of the first child node in the underlying node array.
{@code -1} is returned if {@code this} node is a leaf.

@return Return the index of the first child node in the underlying node
        array, or {@code -1} if {@code this} node is a leaf
jenetics.ext/src/main/java/io/jenetics/ext/util/FlatTreeNode.java