interface problem

[Trass3r]

I got 2 classes which both (indirectly) inherit from a common base class and implement a certain interface I.

Now I need to pass that interface to a function and need to call a function inherited from the base class  inside.
Naturally that function is not present in the interface and thus can't be called, so how to solve that problem?

Adding the function to the interface doesn't work. It doesn't recognize the inherited version and yields "not implemented"