code: func Signal(keyvals []interface{}) string {
  	last := len(keyvals) - 1
  	if last > 0 && keyvals[last-1] == SignalKey {
  		signal, ok := keyvals[last].(string)
  		if ok {
  			return signal
  		}
  	}
  	return ""
  }
docs: // Tried to interpret the logline as a signal by matching the last key-value pair as a signal,
  // returns empty string if no match. The idea with signals is that the should be transmitted to a root logger
  // as a single key-value pair so we avoid the need to do a linear probe over every log line in order to detect a signal.