/* define how to combine observers */
template <Observer Obs1, Observer Obs2, Monitor Mo>
struct CombinedObserver {
 private:
  Obs1 a; /**< First Observer. */
  Obs2 b; /**< Second Observer. */
  Mo mo;  /**< Combination of First and Second Observers' Monitors */

 public:
  void before_timestepping(const view_constgbx gbxs) const {
    a.before_timestepping(gbxs);
    b.before_timestepping(gbxs);
  }

  void after_timestepping() const {
    a.after_timestepping();
    b.after_timestepping();
  }

  unsigned int next_step(const unsigned int t) const {
    return Kokkos::min(a.next_step(t), b.next_step(t));
  }

  bool on_step(const unsigned int t) const {
    return a.on_step(t) || b.on_step(t);
  }

  void at_start_step(const unsigned int t, const view_constgbx,
                     const view_constsupers supers) const {
    a.at_start_step(t, gbxs, supers);
    b.at_start_step(t, gbxs, supers);
  }

  Monitor auto get_monitor() const { return mo; }
};

auto operator>>(const Observer auto obs1, const Observer auto obs2) {
  const Monitor auto mo12 =
      CombinedMonitor{obs1.get_monitor(), obs2.get_monitor()};
  return CombinedObserver{obs1, obs2, mo12};
}

// ... //

/* use a combination of observers */
const Observer auto obs1 = StreamOutObserver(config);
const Observer auto obs2 = TimeObserver(config, dataset);
const Observer auto obs3 = StateObserver(config, dataset);
const Observer auto obs4 = SuperdropsObserver(config, dataset);
const Observer auto observer =
    obs4 >> obs3 >> obs2 >> obs1;  // change this line to change observer
}