Comment 6 for bug 1250401

command

 $ sudo -u root -i
 $ cat /sys/class/backlight/acpi_video0/actual_brightness; echo 3 >> /sys/class/backlight/acpi_video0/brightness; cat /sys/class/backlight/acpi_video0/actual_brightness

will dump

99
99

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that is, before and after the echo, actual_brightness is the same