Inheritance and primary key

I use a super-class S (virtual, in fact) ans three inheritated classes S1, S2
and S3. i wish to have a primary key for all these classes, in order to use
queries as : SELECT * FROM S* WHERE s.id=x
I use a serial ID field for S as a primary key. Sequencing is OK, as any
insertion in any of the four classes increments the counter. But uniqueness
constraint is lost, as i can have duplicated values between differents classes.
And even in the same class for the inherited ones, as primary key propertie of
ID column is lost through inheritance. And i can't specify a unique constraint
on this column, as it is not present in the class definition.
How to solve this problem ?

--
Guillaume Rousse
*******************************************
Iremia -Universit� de la R�union
15 avenue Ren� Cassin, BP 7151
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Mail:Guillaume.Rousse@univ-reunion.fr

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