Extending Session Library in Codeigniter 3.1.8

Hello every one, I have worked with codeigniter for a while now and currency developing an application in the same.
I always find it easy to extend some built in libraries, the email class for instance, but never have I been faced with a situation where I need to extend the session library. Here is the thing, my email sub-class works fine and I have all the extended functionality that I have added, however, when I try extending the session library, it seems my sub-class is not used. It seems codeigniter still loads the in-built session library.

I have searched allover for how to fix this, but I don't seem to get a solution. Anyone with an idea of how to go about this?
Thanks in advance.