adding column constraint

Can someone give an example of how to add a foreign key constraint to an
existing table?  (Pgsql and the man page differ and neither syntax seems
to work.)

I've tried:

alter table mytable add constraint col foreign key references reftable

with no luck.

Tim

-- 
Timothy H. Keitt
National Center for Ecological Analysis and Synthesis
735 State Street, Suite 300, Santa Barbara, CA 93101
Phone: 805-892-2519, FAX: 805-892-2510
http://www.nceas.ucsb.edu/~keitt/