code: def set_target(self, niterations=None, eff_nsamples=None):
          
          if niterations is None and eff_nsamples is None:
              raise ValueError("Must provide a target niterations or "
                               "eff_nsamples")
          if niterations is not None and eff_nsamples is not None:
              raise ValueError("Must provide a target niterations or "
                               "eff_nsamples, not both")
          self._target_niterations = int(niterations) \
              if niterations is not None else None
          self._target_eff_nsamples = int(eff_nsamples) \
              if eff_nsamples is not None else None
docs: Sets the target niterations/nsamples for the sampler.
  
          One or the other must be provided, not both.