RULE for mtime recording

Hi folks,

I'd like to write an update rule, which touches the a mtime field
(=current_timestamp) on normal update - when the mtime field isnt
explicitly set. If the update query explictly sets an mtime value,
this value has to be let through.

my tables look like:

-- base class
CREATE TABLE inode
(
inode_id oid not null default nextval('inode_id_seq'),
mtime timestamp not null default current_timestamp
);

-- example class
CREATE TABLE foo
(
bar text
) INHERITS ( inode );

now if I do

UPDATE foo SET bar = 'xyz' WHERE ...

the mtime should be set to the current time, but on

UPDATE foo SET bar = '123', mtime = '2001-09-11' WHERE ...

we shall have 9/11 as mtime.

Is this possible with rules ?

thx
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