Rule Question

Hope this question is not too stupid but......

I am trying to do something like this

create table cats (a text,b text);

create rule cats_test as on update to cats do set a = new.b;

Can i manipulate column "a" sort of like this...  or is  there a better way.

I would like to do this as the construction of the new.b value is complex and time consuming, so I just want to do it once.  

update cats

set b = something_complex_and_time_consuming(b);    

Thanks

Andrew