class group :object..end
The basic usage is to have only one group and one configuration file,
but this mechanism allows to have more,
for instance to have another smaller group for the options to pass on the command line.
method add : 'a. 'a cp -> unit'a must be lost
    to allow cps of different types to belong to the same group.Double_name if cp#get_name is already used.method write : ?with_help:bool -> string -> unitwrite filename saves all the cps into the configuration file filename.method read : ?obsoletes:string ->
       ?no_default:bool ->
       ?on_type_error:(groupable_cp ->
                       Raw.cp ->
                       (Pervasives.out_channel -> unit) ->
                       string -> Pervasives.in_channel -> unit) ->
       string -> unitread filename reads filename
      and stores the values it specifies into the cps belonging to this group.
      The file is created (and not read) if it doesn't exists.
      In the default behaviour, no warning is issued
      if not all cps are updated or if some values of filename aren't used.
      If obsoletes is specified,
      then prints in this file all the values that are
      in filename but not in this group.
      Those cps are likely to be erroneous or obsolete.
      Opens this file only if there is something to write in it.
      If no_default is true, then raises Missing_cp foo if
      the cp foo isn't defined in filename but belongs to this group.
      on_type_error groupable_cp value output filename in_channel
      is called if the file doesn't give suitable value
      (string instead of int for instance, or a string not belonging to the expected enumeration)
      for the cp groupable_cp.
      value is the value read from the file,
      output is the argument of Config_file.Wrong_type,
      filename is the same argument as the one given to read,
      and in_channel refers to filename to allow a function to close it if needed.
      Default behaviour is to print an error message and call exit 1.
method read_string : ?obsoletes:string ->
       ?no_default:bool ->
       ?on_type_error:(groupable_cp ->
                       Raw.cp ->
                       (Pervasives.out_channel -> unit) -> string -> unit) ->
       string -> unitread_string string reads the content of string
        and stores the values it specifies into the cps belonging to this group.
        This method behaves just like read for the others aspects.
method command_line_args : section_separator:string -> (string * Arg.spec * string) listArg.parse and Arg.usage.section_separator : the string used to concatenate the name of a cp,
      to get the command line option name.
      "-" is a good default.