pg_hba.conf and user files

I am trying to set up pg_hba.conf for PostgreSQL 7.3.  I want to limit
access to a database from a host to the set of users in a file.  The
documentation suggests that this is done as follows:

    host dbname @user_file 10.0.1.12 255.255.255.255 md5

However, the postmaster complains of a syntax error for lines such as
this.  There is in fact a file of the appropriate name (user_file in
same directory as pg_hba.conf), readable by the backend.  It contains
a list of user names, one per line.

Is this the correct setup?  If not, how is this supposed to be
accomplished?

Thanks for your help.

Cheers,
Brook