pub trait RelativeLinkedList<T: Num>: LinkedList<T> {
fn get_first(&self, head: T) -> T;
fn get_last(&self, head: T) -> T;
fn get_size(&self, head: T) -> T;
fn set_first(&mut self, head: T, element: T);
fn set_last(&mut self, head: T, element: T);
fn set_size(&mut self, head: T, size: T);
fn inc_size(&mut self, head: T) { ... }
fn dec_size(&mut self, head: T) { ... }
}