code: func (w *ledgerDriver) Heartbeat() error {
  	if _, err := w.ledgerVersion(); err != nil && err != errLedgerInvalidVersionReply {
  		w.failure = err
  		return err
  	}
  	return nil
  }
docs: // Heartbeat implements usbwallet.driver, performing a sanity check against the
  // Ledger to see if it's still online.