Examples:

There are currently four data files, the second of which has a current background file ÇURRB.PHA and the last of which has no background being subtracted, equivalent to a background file none.

XSPEC> fakeit backa,,none 4  ! Four fake data PHA files are created. The
                               first and second use BACKA.PHA and
                               CURB.PHA for background, and their
                               associated response files for the
                               response.  The third and fourth files
                               will have no background files;
                               the user will be prompted for the
                               response file to be used.  The 4
                               indicates that four data files will
                               be created.