Adding a rule to update a last_changed field

I'm trying to use a rule to maintain a last_changed field.

Using something like:

CREATE RULE rule1 AS
ON UPDATE TO thetable
DO UPDATE thetable SET last_changed = now()
WHERE id = new.id;

This obviously doesn't work as the DO UPDATE triggers the rule
again, thus looping meaninglessly.

Is there a neat way to work around this? I'd like to just assign
now() to the last_changed field before the actual SQL update
code is run.

Thanks

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