Are circular REFERENCES possible ?

Hello !

I have a case where I wanted to do circular REFERENCES, is this
impossible ?

Just an example where it would be useful :

We deliver to the *shops* of our *customers*.
We have therefore two tables : - customers (enterprise, financial information, and so on...) - shop (with a name,
street,phone number, name of manager)
 

Now, each shop REFERENCES a customer so that we know
to which customer belongs a shop.

AND, each customer has a DEFAULT shop for deliveries, i.e. most
customers only have one shop, or a main shop and many small ones.
Therefore a customer should REFERENCES the 'main' or 'default' shop.

Which leads to :

CREATE TABLE shops ( id_shop SERIAL PRIMARY KEY, id_cust integer REFERENCES 
customers, .......)
CREATE TABLE customers ( id_cust SERIAL PRIMARY KEY, id_defaultshop integer 
REFERENCES shops, .......)

But this doesn't work ! Postgres complains like "ERROR:  Relation 
'customers' does not exist"
when creating 'shops'.

Someone told me I should create a third table, ok, but in this case I loose 
the total
control about my logic... Do you have a suggestion ?

Thanks a lot in advance !

Denis