ALTER TABLE SET DEFAULT

Hi,

I couldn't find the right syntax to add a DEFAULT value for a column in
an existing table. I'm actually workin on a migration from MSSQL to
postgres, and the scripts generated by mssql include such statements as:

alter table contacts  add constraint df_contacts_languageid default (0) for languageid;

How do I convert this to a psql statement? I didn't find an example in
the docs or on google.

Thanks for your help!

Raph