1 ADD
  Syntax: ADD id1 [:id1n] [&idb1] id2 [:id2n] [&idb2] id3 [:id3n] [&idb3]
          [scale1] [scale2]                      (default scale = 1.0 1.0)
   where: id1,id2   are the input histogram identifiers
          id3       is the output histogram identifier
          idb1,idb2 are the (optional) input secondary identifiers
          idb3      is the (optional) output secondary identifier

  Adds two histograms (id1, id2)  together to make a third one. To specify
  the secondary  identifier,  precede it by  a `&',  otherwise the default
  will be used.  (Use the  `SET IDB' command  to change  the default). The
  scale factors are optional. To avoid confusion, you should give a `<CR>'
  after the identifiers  or make sure the scale  factors are given as real
  numbers.

  To add a range of  histograms, the primary  identifiers you give for the
  input  and output   histograms must  be the  same,  but you  can specify
  different secondary  identifiers. For example,
  `ADD 300:400&1 300:400&2 300 : 400 & 10' will add all  histograms with
  primary identifiers 300 to 400 and secondary identifiers 1, to  those
  with secondary identifiers 2, putting the  results into  histograms with
  the same  primary identifiers and  secondary  identifier 10. If you give
  primary  identifier 0, the  operation  will  be  performed on  all plots
  with the given secondary identifier.

