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
timestampnot 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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