(Newbie) Restricting inherited methods to operate on element from same subclass

[andy2o]

Hi all,

Sorry if the post's title is confusing... I'll explain:

I have a class, called A say, and N>1 subclasses of A, called 
A1, A2, A3, ..., AN say.

Instances of each subclass can sensibly be joined together with other
instances of the *same subclass*. The syntax of the join method is
identical for each of the N subclasses, so it would make sense to
implement a *single* method called join in the toplevel class A, and
then do:

a = A1()
b = A1()
a.join(b) #I want the join method to be inherited from class A

d = A2()
e = A2()
d.join(e) 

But I want to raise an exception if my code finds:

f = A1()
g = A2()
f.join(g) #should raise exception, as cannot join an 
          #instance of A1 to an instance of A2.

How can I verify in a method defined in class A that the subclasses I
am joining are exactly the same? Or is there a design flaw here I
haven't spotted that makes this a bad idea? Or do I need to code N
join methods?

Many thanks,

Andy