code: func Fatalf(format string, v ...interface{}) {
  	f := extractFields(&v)
  	if f != nil {
  		// Let user add a %v at the end of the message when fields are passed to satisfy go vet
  		l := len(format)
  		if l > 2 && format[l-2] == '%' && format[l-1] == 'v' {
  			format = format[0 : l-2]
  		}
  	}
  	std.OutputF(LevelFatal, 2, fmt.Sprintf(format, v...), f)
  	if l, ok := std.(*logger); ok {
  		if o, ok := l.output.(*OutputChannel); ok {
  			o.Close()
  		}
  	}
  	exit1()
  }
docs: // Fatalf calls the Fatalf() method on the default logger
  //
  // Go vet users: you may append %v at the end of you format when using xlog.F{} as a last
  // argument to workaround go vet false alarm.