1 HB_OPEN
  Syntax: HB_OPEN filename
  where   filename  is the name of the file to open.

  Opens an HBOOK4 file. This command  is not usually necessary if you just
  want to fetch histograms, but is  useful in connection with the commands
  `CDIR' and  `LDIR' (or `SET  DIRECTORY'  and `SHOW  DIRECTORY'). You can
  then find out  what histograms are  in a file and  fetch histograms from
  several subdirectories and give them separate secondary identifiers.

  It is a good idea to set the record length for direct access HBOOK
  version 4 files, if it is known. Otherwise you will get an error
  message and then Mn_Fit will try to open the file again with the
  correct record length.  The default is 1024 words. If your files
  have been made with a different record length, use the `SET RECL'
  command to set the length.  This length will also be used for
  `STORE' commands.

