2 BACK_SUB
  Syntax: BACK_SUB [&]idb [[&]idb2 ...] nmode
   where: idb     is the secondary identifier for the background
                  subtracted plot
          nmode   = 1 means just subtract the background
                  = 2 means divide by it also

  Makes a  background subtracted  histogram from the  last fit. You must
  use the `SET BACKGROUND' command beforehand to specify which functions
  are the  background. You will be  prompted for  which mode you want to
  use, and what secondary identifier you wish to give the result. If you
  are fitting more  than 1 histogram  simultaneously,  you should give a
  secondary identifier for each of them.

