I am using FAMIXFile and FAMIXFolder for a project. There are a few things that I've improved.

FAMIXAbstractFile defines a variable for belongsTo, I renamed this variable to parentFolder because AFAIK belongsTo is always derived information. I've implemented parentFolder as a FMMutlivalueLink. The opposite is childrenFileSystemEntities so that we've autoupdating back links. 

This change has made make existing files and folders inst vars and their accessors redundant. The selectors folder and files on a FAMIXFolder still exist but they are computed from childrenFileSystemEntities. IMO, the resulting solution is much more clean and more expressive. 

Let me know if there's a conceptual regression or some cases not covered.

Usman