ledger_common.go
func getLedger() (*ledger.Ledger, error) {
	var err error
	if device == nil {
		device, err = ledger.FindLedger()
	}
	return device, err
}
func getLedger() (*ledger.Ledger, error) {
	var err error
	if device == nil {
		device, err = ledger.FindLedger()
	}
	return device, err
}
// getLedger gets a copy of the device, and caches it