1 AVERAGE
  Syntax: AVERAGE id1 [:id1n] [&idb1] id2[:id2n] [&idb2] id3[:id3n] [&idb3]
   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

  Makes a weighted average of the contents of two histograms (id1, id2) 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).

  To average  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,
  `AVERAGE 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.

