def set_target(self, niterations=None, eff_nsamples=None):
        """Sets the target niterations/nsamples for the sampler.

        One or the other must be provided, not both.
        """
        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
Sets the target niterations/nsamples for the sampler.

        One or the other must be provided, not both.