Using nested controllers

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Hello,

First time here, just started using CI and enjoying it.

Issue:
I'm trying to use nested controllers, something like this:
controllers/welcome.php
controllers/private/welcome.php
controllers/private/students/marks.php
controllers/private/management/marks.php

Sure, I can avoid this by trying this structure:
/controllers/welcome.php
/controllers/students/marks.php
/controllers/management/marks.php

But, could I achieve my first goal? How should my routes.php look like?

To reach the controllers/private/welcome.php it works with:

Code:

$route['private'] = "private/welcome";

But to go down on more level, I can't pull it off.

Thank you.