Dir of an Include File

I have three files:

start.au3  - I double click this file to start the program, it includes "lib/func.au3"
lib/func.au3 - This file runs program.exe by calling Run("...")
lib/bin/program.exe


In order to execute program.exe from func.au3, I need the path to the executable. @ScriptDir returns the path to start.au3, but that's not what I want. I could use @ScriptDir & "/lib/bin/program.exe". However, the result depends on where the library was included from - but I want to be able to include the library from anywhere.

Any ideas how I can get the path to the program?