code: def zero_weight_obs_names(self):
          
          self.observation_data.index = self.observation_data.obsnme
          groups = self.observation_data.groupby(
                  self.observation_data.weight.apply(lambda x: x==0.0)).groups
          if True in groups:
              return list(self.observation_data.loc[groups[True],"obsnme"])
          else:
              return []
docs: get the zero-weighted observation names
  
          Returns
          -------
           zero_weight_obs_names : list
               a list of zero-weighted observation names