inheritance problems

I have a very general table, Orders, from which three other tables are
children. Orders has only two columns cid, oid, have primary key(oid,
cid). I want all the children to also have this primary key, and I want
uniqness of the primary key enforced accross all children. this was the
reason I decided to use inheritance, however I can insert the same entry
in two children and then look at the duplicates in the Orders :( is it
possible, and if so, how is it done; to have unique primary keys accross
all the children.

Matthew Berardi
WeilPublishing