What is "Directory_Search.nsh"

I have inherited a fairly large (at lest to me), installer. I need to maintain this for distribution internally in my company. I have most things figured out but there is one line in the .nsi file I cant figure out, it is the line:

!include Directory_Search.nsh

does anyone know where I can get this? I assume it is a plug in or addon of some sort.

Thanks In Advance
Troy

That's not a common header file I've heard of. It's probably an internal file.