Get Key's Controller

I'm trying to get the name of the controller to which belongs a key.
In my example I select an object in my scene and run the script.

c = bezier_position () -- create and assign new controller
$.pos.controller = c
k = addNewKey c 15f

The script added a position controller to the selected object and created a position key at frame 15.

Now, I can get several information about the key properties like

on which frame it is situated:
k.time

the position determines:
k.value

and so on..

What I'm looking for is a way to get the information of the controller itself.

Something like:
k.controller

But this unfortunately doesn't exist. :)
Is there a way to get this information?