function tail() {
        assert(!$this->isEmpty());
        return $this->copyFromContext($this->head->next()->next());
    }
function tail() {
        assert(!$this->isEmpty());
        return $this->copyFromContext($this->head->next()->next());
    }
function tail ( ) { assert ( ! $ this -> isEmpty ( ) ) ; return $ this -> copyFromContext ( $ this -> head -> next ( ) -> next ( ) ) ; }
Extract the elements after the first of a list, which must be non-empty.
@return LinkedList
Extract the elements after the first of a list which must be non - empty .