code: def field_keyword_for_the_layer(self):
          
          layer_purpose_key = self.step_kw_purpose.selected_purpose()['key']
          if layer_purpose_key == layer_purpose_aggregation['key']:
              return get_compulsory_fields(layer_purpose_key)['key']
          elif layer_purpose_key in [
                  layer_purpose_exposure['key'], layer_purpose_hazard['key']]:
              layer_subcategory_key = \
                  self.step_kw_subcategory.selected_subcategory()['key']
              return get_compulsory_fields(
                  layer_purpose_key, layer_subcategory_key)['key']
          else:
              raise InvalidParameterError
docs: Return the proper keyword for field for the current layer.
  
          :returns: the field keyword
          :rtype: str