code: func (s folderDeviceSet) set(dev protocol.DeviceID, folder string) {
  	devs, ok := s[folder]
  	if !ok {
  		devs = make(map[protocol.DeviceID]struct{})
  		s[folder] = devs
  	}
  	devs[dev] = struct{}{}
  }
docs: // set adds the (dev, folder) pair to the set