Extending Controller

[eluser]dieter[/eluser]
I have a similar issue:

Situation:

Code:

MY_Controller extends Controller () { ... }

Code:

class Controller_A extends MY_Controller
{
    function Controller_A()
    {
        parent::MY_Controller();
    }
}

Works fine

Code:

class Controller_B extends Controller_A
{
    function Controller_B()
    {
        parent::Controller_A();
    }
}

Doesn't work:
Fatal error: Class 'Controller_A' not found in (...)

Any ideas or suggestions to solve this?

Thanks,
Dieter