Trigger for modification timestamp column

Hello!

I am trying to write a trigger for updating a modification column and am
having some trouble getting it to behave as I want.

The trigger should set the column to the supplied value if it is set in the
UPDATE statement and to the current timestamp [NOW()] if it is not. The
problem is that I don't know how to check if the column is set or not. I can
check the column's value for NULL but that doesn't work if I want the column
to accept NULL values (meaning "unmodified").

I would like something like:
CREATE FUNCTION update_modified()
    RETURNS TRIGGER AS $$
        BEGIN
            IF NOT isset(NEW.modified) THEN
                NEW.modified = NOW();
            END IF;
            RETURN NEW;
        END;
    $$ LANGUAGE 'plpgsql';

Does anyone know how to do this?

Thanks in advance!

/ Johan
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