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struct FnHolder {
  virtual json call(std::vector<json>);
};
template <typename Ret, typename...Arg>
struct FnHolderImpl : FnHolder {
  template<typename T>
  T fromjson(json);
  template<>
  std::string frojson(json); /* this typechecks etc */
  // here other impls

  template<typename T>
  json tojson(T); /* ....yada yada with all the impls */

  Ret (*_func)(Arg...);
  FnHolderImpl(Ret (*func)(Arg...)) : _func(func) {} // store func for later
  json call(std::vector<json> args) {
    if (args.size() != sizeof...(Arg))
       throw error;
    std::vector<std::any> xs = { fromjson<Arg>(fetch_back(args))... };
    Ret value = _func(any_ref_cast<Arg>().do_cast(fetch_back(xs)));
    return tojson(value);
  }
};
std::unordered_map<std::string /*name*/, FnHolder*> container;
template <typename Ret, typename...Arg>
void register(std::string name, Ret (*func)(Arg...)) {
  container[name] = new FnHolderImpl(func);
}