foreign key constraint

Is there any way to add a foreign key constraint after a table is created? I got this error
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PostgreSQL said: ERROR: parser: parse error at or near "FOREIGN"
Your query:
ALTER TABLE testUsrEmails ADD CONSTRAINT FOREIGN KEY (usr_id) REFERENCES testUsrs (usr_id)
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OR, how is it possible to add foreign key constraints, ('cross constraints'), between two tables.
I tried it in a transaction, but it didn't work. The first constraint in the table definition
was DEFERRABLE INITIALLY DEFERRED, but I don't think the parser cared :-) I got this error:

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PostgreSQL said: ERROR: Relation "testusremails" does not exist (it's the next table in the
creation list)
Your query:
CREATE TABLE testUsrs(
usr_id serial NOT NULL PRIMARY KEY,
usr_email_id_pri int4 NOT NULL,
login varchar(32) NOT NULL UNIQUE,
hashed_pw text NOT NULL,
sur_name text NOT NULL,
first_name text NOT NULL,
middle_name text DEFAULT 'none' NOT NULL,
created timestamp DEFAULT CURRENT_TIMESTAMP NOT NULL,
FOREIGN KEY (usr_email_id_pri) REFERENCES testUsrEmails (usr_email_id) DEFERRABLE INITIALLY
DEFERRED);
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