log.go
func (b *Backend) Logger(subsystemTag string) Logger {
	return &slog{LevelInfo, subsystemTag, b}
}
func (b *Backend) Logger(subsystemTag string) Logger {
	return &slog{LevelInfo, subsystemTag, b}
}
// Logger returns a new logger for a particular subsystem that writes to the
// Backend b.  A tag describes the subsystem and is included in all log
// messages.  The logger uses the info verbosity level by default.