default ISeq<T> flattenedNodes() {
		return root().breadthFirstStream().collect(ISeq.toISeq());
	}
Return the whole flattened tree values in breadth-first order. This is
equivalent to
{@snippet lang="java":
final ISeq<T> seq = getRoot().breadthFirstStream()
    .collect(ISeq.toISeq());
}

@return the flattened tree values in breadth-first order
jenetics.ext/src/main/java/io/jenetics/ext/util/FlatTree.java