Hierarchical directories are also connective structures in two dimensions.
Which basically means that people aren't thinking of hierarchical directories as a special case of anything else. But we are, and we hope you will too. We are familiar with four dimensions that have distance. But every schoolkid knows that these three dimensions of space were generalized by Einstein into a fourth dimension of time. It is less well known that they are further generalized by mathematicians into such systems as Hilbert-space.