docs: def to_json(self, indent=4):
          """
          Serialize metas and reference attributes to a JSON string.
  
          Keyword Arguments:
              indent (int): Space indentation, default to ``4``.
  
          Returns:
              string: JSON datas.
          """
          agregate = {
              'metas': self.metas,
          }
  
          agregate.update({k: getattr(self, k) for k in self._rule_attrs})
  
          return json.dumps(agregate, indent=indent)