docs: def _sampler_n_samples(self, n_samples):
          """ Return (sampler, n_samplers) tuples """
          sampler_indices = self.rng_.choice(range(len(self.samplers)),
                                             size=n_samples,
                                             replace=True,
                                             p=self.weights)
          return [
              (self.samplers[idx], freq)
              for idx, freq in itemfreq(sampler_indices)
          ]