code: func (w *trezorDriver) Heartbeat() error {
  	if _, err := w.trezorExchange(&trezor.Ping{}, new(trezor.Success)); err != nil {
  		w.failure = err
  		return err
  	}
  	return nil
  }
docs: // Heartbeat implements usbwallet.driver, performing a sanity check against the
  // Trezor to see if it's still online.