code: func Register(rk string, f Formatter) {
  	if rk == "" {
  		panic("cannot register empty key")
  	}
  
  	registry.Lock()
  	defer registry.Unlock()
  
  	if _, ok := registry.r[rk]; ok {
  		panic(fmt.Errorf("key %q is already registered", rk))
  	}
  	if registry.r == nil {
  		registry.r = map[string]Formatter{}
  	}
  	registry.r[rk] = f
  }
docs: // Register registers a Formatter implementation for given format.
  //
  // If the supplied key is already registered, Register will panic.