docs: def add_dat_file(filename, settings, container=None, **kwargs):
      """ Read a RES2DINV-style file produced by the ABEM export program.
      """
      # each type is read by a different function
      importers = {
          # general array type
          11: _read_general_type,
      }
  
      file_type, content = _read_file(filename)
  
      if file_type not in importers:
          raise Exception(
              'type of RES2DINV data file not recognized: {0}'.format(file_type)
          )
  
      header, data = importers[file_type](content, settings)
  
      timestep = settings.get('timestep', 0)
  
      # add timestep column
      data['timestep'] = timestep
  
      if container is None:
          container = ERT(data)
      else:
          container.data = pd.concat((container.data, data))
  
      return container