simulate inheritance with generic package

[WATANABE Katsuhiro <katsu@watanabe.name>]

I'd like to know how to simulate inheritance in OO with
generic package in classic Ada, which has no tagged type.

An ancient OO book(*) instructs a way of inheritance
implementation in Ada by introducing has-a reference from
child class to parent class. And it says their functions
should be linked, that is to say, any call to the child
instance must be delegated to the parent instance. And
the description goes on as follows:

> If you use generic package this linkage can be made
> by using a specific routine as the relevant (generic)
> parameter when instantiating the package.

But, what does 'a specific routine' means?

As I am a Smalltalk programmer, I have limited knowledge
on Ada and its instanciation mechanism of generic package.
So, I'd appreciate your help showing concretely what 
'a specific routine' is.

(*)
Ivar Jacobson;
Object-Oriented Software Engineering
--- A Use Case Driven Approach ---;
Addison-Wesley;
1994

-- 
WATANABE Katsuhiro