docs: def to_json(self, lev=0, indent=None):
          """
          Serialize the content of this instance into a JSON string.
          
          :param lev: 
          :param indent: Number of spaces that should be used for indentation 
          :return: 
          """
          if lev:
              return self.to_dict(lev + 1)
          else:
              return json.dumps(self.to_dict(1), indent=indent)