How do I extend a core library without duplicating its code?

in application/config/config.php   set $config['subclass_prefix'] = 'MY_';
if extend core controller: create application/core/MY_Controller.php 

PHP Code:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class MY_Controller extends CI_Controller { 

    public function __construct() {

        parent::__construct();    

        // do some stuff   
    }
} 


and your controllers extends MY_Controller:

PHP Code:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class User extends MY_Controller { 

    public function __construct() {

        parent::__construct();    
    }
}