Tsearch2: casting text to tsquery

Hello,

I have a question that arose while using tsearch on a large database
of book information.

In order to inject unstemmed + stemmed versions of searchstrings into
a query we want to cast an expression of type text to tsquery.
Unfortunately

SELECT 'word'::tsquery

(where the literal 'word' is of type 'unknown' seemingly) works just
fine while e.g.

SELECT ('word' || '&fullword')::tsquery

or, simpler

SELECT 'word&fullword'::text::tsquery

gives

ERROR: cannot cast type text to tsearch2.tsquery

How can we work around that? Thanks for any help

Sincerely yours
Alexander Presber