implicit lock in RULE ?

Hallo,

Are tables, that are accessed by a rule, implicitely locked ?
I tried

CREATE RULE new_vc_thread AS ON INSERT TO images
WHERE new.vc_thread = 0
DO
(
BEGIN;
LOCK vc_threads;
INSERT INTO vc_threads(name) VALUES(new.name);
UPDATE images SET vc_thread = currval('vc_threads_id_seq') WHERE vc_thread=0;
COMMIT;
);

but got a syntax error - OK.

But how can I work around it ?
See my code, especially the comments in the CREATE RULE statement:
-----------------------------------------------------------------------
/*-----------------------------------------------
    Scenario:
------------------------------------------------*/
CREATE TABLE vc_threads(
    id serial primary key,
    name varchar(100) DEFAULT NULL
);

INSERT INTO vc_threads(id,name) VALUES(0,'DEFAULT VC_THREAD');


CREATE TABLE images(
    id serial PRIMARY KEY,
    vc_thread int REFERENCES vc_threads(id) NOT NULL DEFAULT 0,
    name varchar(80) NOT NULL
);

/*-----------------------------------------------
    The RULE in question:
I want to have a new vc_thread, any time an image
without specified "vc_thread" column is inserted.
------------------------------------------------*/

CREATE RULE new_vc_thread AS ON INSERT TO images
WHERE new.vc_thread = 0
DO
(
-- the insert on "images" is now already made
INSERT INTO vc_threads(name) VALUES(new.name);
-- this insert on "vc_threads" implicitly calls nextval('vc_threads_id_seq'),
-- such that
UPDATE images SET vc_thread = currval('vc_threads_id_seq') WHERE vc_thread=0;
-- should set images.vc_thread to the new vc_thead's id
-- UNLESS (!!) any concurrent client calls nextval('vc_threads_id_seq') between the
-- INSERT and the UPDATE. Can that happen ?
);

-- Tests:
INSERT INTO images(name) VALUES('bild1');
-- INSERT 101487 1
INSERT INTO images(vc_thread,name) VALUES(null,'bild2');
-- psql:test.sql:27: ERROR:  ExecInsert: Fail to add null value in not null
attribute vc_thread
INSERT INTO images(vc_thread,name) VALUES(0,'bild3');
-- INSERT 101489 1
INSERT INTO images(vc_thread,name) VALUES(1,'bild4');
-- INSERT 101491 1
INSERT INTO images(id,name) VALUES(15,'bild5');
-- INSERT 101492 1

-- works correct, but is it safe ?
--------------------------------------------------------------

Sorry, I couldn't find the answer in the docs.

Thanks,  Fritz