@Override
default <U> Tree<U> map(Function<? super T, ? extends U> mapper) {
Objects.requireNonNull(mapper, "mapper is null");
return isEmpty() ? Empty.instance() : TreeModule.Map.apply((Node<T>) this, mapper);
}
static <T, U> Node<U> apply(Node<T> node, Function<? super T, ? extends U> mapper) {
final U value = mapper.apply(node.getValue());
final List<Node<U>> children = node.getChildren().map(child -> Map.apply(child, mapper));
return new Node<>(value, children);
}