/* define an Observer */
template <typename Obs>
concept Observer = requires(Obs obs, unsigned int t, view_constgbx gbxs,
                            view_constsupers supers) {
  { obs.before_timestepping(gbxs) } -> std::same_as<void>;
  { obs.after_timestepping() } -> std::same_as<void>;
  { obs.next_step(t) } -> std::convertible_to<unsigned int>;
  { obs.on_step(t) } -> std::same_as<bool>;
  { obs.at_start_step(t, gbxs, supers) } -> std::same_as<void>;
  { obs.get_monitor() };  // should return a Monitor type
};