code: def get_model_abbrev(model_obj):
      
      # Get the 'display name' for our model.
      model_type = model_obj.model_type
      # Find the model abbreviation for this model's display name.
      for key in model_type_to_display_name:
          if model_type_to_display_name[key] == model_type:
              return key
      # If none of the strings in model_type_to_display_name matches our model
      # object, then raise an error.
      msg = "Model object has an unknown or incorrect model type."
      raise ValueError(msg)
docs: Extract the string used to specify the model type of this model object in
      `pylogit.create_chohice_model`.
  
      Parameters
      ----------
      model_obj : An MNDC_Model instance.
  
      Returns
      -------
      str. The internal abbreviation used for the particular type of MNDC_Model.