Trigger vs rule

I am in the process of creating a database to keep of my car's fuel costs
and mileage. I am having a little bit of trouble deciding if I need to use
a rule to do what I want or a trigger. I have created a SQL function that
will update the mileage field, but when I try and create a trigger for it,
it says it needs to return OPAQUE, but if i recreate my SQL function it
won't accept OPAQUE as a return type.

Any Ideas?

--
Richard Nairn
Nairn Consulting
** Email ren(at)spans(dot)gscc(dot)nrcan(dot)gc(dot)ca **
** nairnr(at)cadvision(dot)com **
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