tablename type?

The PL/PGSQL documentation contains at least two examples of functions
which take an argument of type "tablename", which then serves as a
table name in a query. Here's one of those examples:

> CREATE FUNCTION concat_selected_fields(tablename) RETURNS text AS '
> DECLARE
> in_t ALIAS FOR $1;
> BEGIN
> RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
> END;
> ' LANGUAGE plpgsql;

Typing this at the psql prompt, however, simply results in the
following message:

ERROR: type tablename does not exist

Is the documentation incorrect, or is there something wrong with my
PostgreSQL 7.4.3 installation?

If the documentation is incorrect, is there another way to achieve
this, or do I have to use a varchar argument and construct and execute
a dynamic command?

DES
--
Dag-Erling Smørgrav - des(at)des(dot)no