[QUESTIONS] builtin lo_unlink(int4)? why int4 not oid?

Hello all,

e.g.

    I want to delete a large object with this table

CREATE TABLE image (
    name            text,
    raster          oid
);

-- from programmer's guide

in the psql

foo=> select lo_unlink(raster) from image;
ERROR:  function int4(oid) does not exist

Why builtin "lo_unlink" is defined as accepting int4 not oid?  Then do I
have to do
foo=> select lo_unlink(int4(oid_text(raster))) from image;
OR
define "raster" as int4?  I don't think all these are good idea...  Then
how to delete "lo" in the "psql"?

Best Regards,
C.S.Park