First one, related to what I was wandering about, is mathematical
definitions and relationships. Take the picture of the first volume of
van der Waerden's Albebra (I have the german edition and will keep the
german words). We speak about links between notions presented in a
linear order: the order of the chapters:
Galois theory chapter is the descendant of concepts from
Vektorräume, Fortsetzung der Gruppentheorie and Körpertheorie chapters;
Vektorräume and Gruppentheory being siblings ("children" of Ringe und
Körper) while Körpertheorie is a grand-child of Vektorräume.
Example of multiple parents, and not at the same level.
This seems to be possible to recreate with befs, but I have to say: this particular problem is too specific to be solved with something like a filesystem. One should write a fileserver only when that would allow the user to use multiple familiar tools to work on the task. With your example it looks like the only thing the user would need here is the list of related topics along with the description of their relationship so that a related topic could be chosen and read. You can use a relational database for storage of the content along with the list of related topics and a program that can browse that database and follow the links. A user would never even think about doing something like 'cp ./Galors theory/parents ./Galors theory/children, even though the approach of implementing that in a fileserver allows that.