code: func Use(c context.Context, cfg *Config) context.Context {
  	config := Config{}
  	if cfg != nil {
  		config = *cfg
  	}
  	if strings.Contains(config.Prefix, ":") {
  		panic("forbidden character ':' in Prefix")
  	}
  	if config.SecretLen == 0 {
  		config.SecretLen = 32
  	}
  	if config.Entropy == nil {
  		config.Entropy = rand.Reader
  	}
  	return secrets.SetFactory(c, func(ic context.Context) secrets.Store {
  		return &storeImpl{config, ic}
  	})
  }
docs: // Use injects the GAE implementation of secrets.Store into the context.
  // The context must be configured with GAE datastore implementation already.