default Optional<T> nextSibling() {
		return parent().flatMap(p -> p.childAfter(self()));
	}
Return the next sibling of {@code this} node in the parent's children
array, or {@code null} if {@code this} node has no parent, or it is the
last child of the paren. This method performs a linear search that is
{@code O(n)} where n is the number of children; to traverse the entire
array, use the iterator of the parent instead.

@see #childStream()
@return the sibling of {@code this} node that immediately follows
        {@code this} node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java