default Optional<T> previousSibling() {
		return parent().flatMap(p -> p.childBefore(self()));
	}
Return the previous sibling of {@code this} node in the parent's children
list, or {@code Optional.empty()} if this node has no parent or is the
parent's first child. This method performs a linear search that is O(n)
where n is the number of children.

@return the sibling of {@code this} node that immediately precedes this
        node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java