Create index on subfield returned by function that returns base type with sub fields

Hello,

Is is possible to create an index on a field on a function that returns a data type that contains subfields?

It is possible to do this:
create index indx_test
on address
(sp_address_text_to_template(address_text))
where (sp_address_text_to_template(address_text)).city_name = 'some_city_on_some_planet';

,but, I would like to create the index without the partial clause to index results only from sp_address_text_to_template(address_text)).city_name.

Is this possible? How would I write the statement?

Thanks,

Desmond.