code: def __apply_nested_option(self, option_name, field_names, set_operation):
          
          # Split nested field names on the first dot.
          nested_fields = [name.split('.', 1) for name in field_names if '.' in name]
          # Partition the nested field names by parent field.
          nested_options = defaultdict(list)
          for parent, nested_names in nested_fields:
              nested_options[parent].append(nested_names)
          # Apply the nested field options.
          for key, options in iter(nested_options.items()):
              new_options = self.set_class(options)
              original_options = getattr(self.declared_fields[key], option_name, ())
              if original_options:
                  if set_operation == 'union':
                      new_options |= self.set_class(original_options)
                  if set_operation == 'intersection':
                      new_options &= self.set_class(original_options)
              setattr(self.declared_fields[key], option_name, new_options)
docs: Apply nested options to nested fields