avoid circular references

Hi there,

I've got a table 'objtable' of objects which primary id is 'idobj' of type
serial.

objtable :
       idobj      objname       1          nameone       2          nametwo        3          namethree       ...
      
 
Each object can contain one or many other objects from the same
table, so I guess this relationship can be expressed with something
like :
 CREATE TABLE contains (id SERIAL PRIMARY KEY NOT NULL,   containerid INTEGER REFERENCES objtable(idobj),     contentid
INTEGERREFERENCES objtable(idobj) );     
 
What I would like is to be able to avoid circular references :      
       - an object can't contain itself.              - an object can't contain one of its containers.

So I'd like to know how to create additionnal integrity constraints 
to solve this problem, and especially what syntax I should use. 

I thought about creating a rule but maybe the best is a trigger
(I'm not sure I really understand the difference) if I see
what happens when I create new tables. But what can I do in
my trigger to have PostgreSQL understand there's an integrity
violation ?

Thanks in advance for any help on this.

Jerome Alet

-- 
"A non-free program is a predatory social system that keeps people 
in a state of domination and division, and uses the spoils to 
dominate more." - RMS