multiple extensions to a class

[Humpty Dumpty]

Hello, I'm experimenting with different ways of extending a class (for a
plug-ins framework for a GUI) with more than one extension when some of
these extensions need to collaborate, but others mustn't know about each
other.

I.e.,  if I have a class A, and I want to add a block of functionality, I
can derive it into a B that adds that fucntionality. If I want to add more
functionality, I can derive B into a C. But if I want to add a third bit of
functionality D directly to A, such that D knows nothing about B or C, I
won't be able to instantiate an object that has all three extensions, namely
an "ABCD". It will either be an "ABC" or an "AD".

One possibility would be to define the extensions B,C and D as functions and
add those functions to A with new.instancemethod. Any other ideas on how to
do this? E.g. is it possible to *add* base classes to a class? I could add
the extensions by adding them as base classes to A.

Thanks,
Oliver