code: func mapDevices(devices []protocol.DeviceID) map[protocol.DeviceID]struct{} {
  	m := make(map[protocol.DeviceID]struct{}, len(devices))
  	for _, dev := range devices {
  		m[dev] = struct{}{}
  	}
  	return m
  }
docs: // mapDevices returns a map of device ID to nothing for the given slice of
  // device IDs.