I don't know why there are many external files that are not existed on the pre-written functions of AutoIt software..
The examples in this page are telling to include "MsgBoxConstants.au3" but I doesn't have one:
http://www.autoitscript.com/autoit3/docs/functions/GUIGetMsg.htm
Where can I download the TRUSTED and the official "MsgBoxConstants.au3" file?