code: def _sample_item(self, **kwargs):
          
          t = self.t_
  
          # Update instrumental distribution
          self._calc_inst_pmf()
  
          if self.record_inst_hist:
              inst_pmf = self._inst_pmf[:,t]
          else:
              inst_pmf = self._inst_pmf
  
          # Sample label and record weight
          loc, stratum_idx = self.strata.sample(pmf = inst_pmf)
          weight = self.strata.weights_[stratum_idx]/inst_pmf[stratum_idx]
  
          return loc, weight, {'stratum': stratum_idx}
docs: Sample an item from the pool according to the instrumental
          distribution