Add a command to the submit file to allow variable (macro) arguments
    to be passed to the executable.

def add_var_arg(self,arg_index):
    """
    Add a command to the submit file to allow variable (macro) arguments
    to be passed to the executable.
    """
    try:
      self.__var_args[arg_index]
    except IndexError:
      if arg_index != self.__arg_index:
        raise CondorDAGJobError, "mismatch between job and node var_arg index"
      self.__var_args.append('$(macroargument%s)' % str(arg_index))
      self.add_arg(self.__var_args[self.__arg_index])
      self.__arg_index += 1