Re: exclusion constraint question

    CHECK (l_mug_id <> c_mug_id AND l_mug_id <> r_mug_id AND c_mug_id
<> r_mug_id),
    EXCLUDE USING gist (l_mug_id WITH <>, c_mug_id WITH <>, r_mug_id
WITH <>) -- Not working as expected (or my expectations are wrong).
);

And some data:

INSERT INTO shelves VALUES (1,    7,    2,    1);
INSERT INTO shelves VALUES (2,    3, null, null);
INSERT INTO shelves VALUES (3, null,    1,    4);
INSERT INTO shelves VALUES (4,    4,    5, null);

Any suggestions would be appreciated.