docs: def _sample_item(self, **kwargs):
          """Sample an item from the pool according to the instrumental
          distribution
          """
          t = self.t_
          if 'fixed_stratum' in kwargs:
              stratum_idx = kwargs['fixed_stratum']
          else:
              stratum_idx = None
          if stratum_idx is not None:
              # Sample in given stratum
              loc = self.strata._sample_in_stratum(stratum_idx, replace=False)
              # Record instrumental distribution
              if self.record_inst_hist:
                  self.inst_pmf_[stratum_idx,t] = 1
          else:
              # Choose stratum based on instrumental distribution
              self._calc_inst_pmf()
  
              if self.record_inst_hist:
                  inst_pmf = self.inst_pmf_[:,t]
              else:
                  inst_pmf = self.inst_pmf_
  
              loc, stratum_idx = self.strata.sample(pmf = inst_pmf, replace=False)
  
          return loc, 1, {'stratum': stratum_idx}