Private type definition must be definite : but why ?

Hello, hello...

I'm trying to express something I can't express with Ada semantic :

> package Xxx
>   type Data_Row_Type is private;
>   ...
> private
>   type Data_Row_Type is new String;
>   ...
> end Xxx;

Bu I cannot do that, beceause a private type definition must be
definite.

While I can do

> package Xxx
>   type Data_Row_Type is new String;
>   ...
> end Xxx;

I would like to understand the reason of this restriction. There is a
user view (the public part) and a compiler view (public part + private
part). So what's the matter if I want the user not to rely on the fact
that Data_Row_Type is a new String type, but still want the compiler
to know it. Why is it not allowed ?

What does justify the obligation for a private type to be definite ?

I do not see any reason at the time.

Thanks and good times to you

Yannick