--Is there a controller hierarchy ?

[eluser]Unknown[/eluser]
Hi I am a newbie to CI. I created a controller which extends the Controller:

class BaseSessionController extends Controller {
// some logic here
}

Then I tried to create another controller class which extends the BaseSessionController:

class SubController extends BaseSessionController {
// some logic here
}

These two controllers are in two different php files -- both in the controllers folder. It seems that BaseSessionController could not be recognized by SubController . I am wondering if I have to manually include the BaseSessionController script file in my SubController script, using require() or require_once().