get the current input file's directory



I mainly code my scripts with Maxima for Windows. But the following problem is 
general

I have many scripts which load a package "MyLib.mac". Both the scripts and 
MyLib.mac are in the folder "MyDir" which is not one of default file_search 
directories.
Adding MyDir to file_search_maxima isn't an option because MyDir is fragile: 
it can change later on and/or be copied to another computer (either Linux or 
Windows). So the full path of MyDir is far from a constant string. Of course 
you don't want to edit everytime the following line
	load("d:\\dir1\\dir2\\MyDir\\MyLib")$
The only solution I can see is to retrieve (at runtime) the directory of the 
input script file, the one currently opened in wxMaxima.
In Mathematica you simply would do
	Get["MyLib.mac", Path -> {NotebookDirectory[]}]

There isn't such a command in Maxima/wxMaxima, is it?
Can some low-level trick in Lisp do the job?

PV