Problem with alter table (creating a foreing key post facto)

A curious problem with 7.3.1 : I wanted to add a foreing key constraint to
a table already created and populated. No way, according to Postgres :

Welcome to psql 7.3.1, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

essai=# \d tablea
Table "public.tablea"
Column | Type | Modifiers
--------+---------+----------------------------------------------------------
keya | integer | not null default nextval('public.tablea_keya_seq'::text)
vala | text |
Indexes: tablea_pkey primary key btree (keya)

essai=# \d tableb
Table "public.tableb"
Column | Type | Modifiers
--------+---------+----------------------------------------------------------
keyb | integer | not null default nextval('public.tableb_keyb_seq'::text)
refa | integer | not null
valb | text |
Indexes: tableb_pkey primary key btree (keyb)

essai=# alter table tableb add constraint lien_a_a foreing key (refa)
references tablea (keya);
ERROR: parser: parse error at or near "foreing" at character 44
essai=# alter table tableb add foreing key (refa) references tablea (keya);
ERROR: parser: parse error at or near "(" at character 36

WTF ????

Any ideas ?

Sincerely,

Emmanuel Charpentier