code: func declNative(name string, f nativeFunc) {
  	if nativeFuncs[name] != nil {
  		panic(fmt.Sprintf("native function %q is already registered", name))
  	}
  	nativeFuncs[name] = f
  }
docs: // declNative registers a native function during init() time.
  //
  // Panics if such function has already been registered.