docs: def _normalize_nested_options(self):
          """Apply then flatten nested schema options"""
          if self.only is not None:
              # Apply the only option to nested fields.
              self.__apply_nested_option('only', self.only, 'intersection')
              # Remove the child field names from the only option.
              self.only = self.set_class(
                  [field.split('.', 1)[0] for field in self.only],
              )
          excludes = set(self.opts.exclude) | set(self.exclude)
          if excludes:
              # Apply the exclude option to nested fields.
              self.__apply_nested_option('exclude', excludes, 'union')
          if self.exclude:
              # Remove the parent field names from the exclude option.
              self.exclude = self.set_class(
                  [field for field in self.exclude if '.' not in field],
              )
          if self.opts.exclude:
              # Remove the parent field names from the meta exclude option.
              self.opts.exclude = self.set_class(
                  [field for field in self.opts.exclude if '.' not in field],
              )