Examples:

It is assumed that there are currently three data sets:

XSPEC> backgrnd a,b,c      ! New files for background subtraction are given for all three 
                             data sets.

XSPEC> backgrnd 2 none     ! No background subtraction will be done for the second data set.

XSPEC> backgrnd,,d/        ! d.pha becomes the background for the second data set.