default int siblingCount() {
		return parent().map(Tree::childCount).orElse(1);
	}
Return the number of siblings of {@code this} node. A node is its own
sibling (if it has no parent or no siblings, this method returns
{@code 1}).

@return the number of siblings of {@code this} node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java