default Optional<T> firstChild() {
		return childCount() > 0
			? Optional.of(childAt(0))
			: Optional.empty();
	}
Return the first child of {@code this} node, or {@code Optional.empty()}
if {@code this} node has no children.

@return the first child of this node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java