We do, it's called "the file system". Folders and directories galore. If you dont like folder icons and rectangular windows, navigate with MC. The problem is n

We do, it's called "the file system". Folders and directories galore. If you dont like folder icons and rectangular windows, navigate with MC. The problem is not trees, they are readily available in many formats. The problem is schemas. If there are no rules on the branches everything becomes "Old_stuff" or "important_work" or whatever people do to their document folders as the tide turns. You need trees layered over trees to provide some structure and get that sweet workflow QC. Graph-homomorphisms between trees that is, or slice categories over whatever structure you need to maintain. Trees (or graphs) in semantic/syntactic relationships stacked as high as you can muster. Usually this is presented as a two-layered structure-tree+data-tree system in end user applications, with a fixed semantic tree depending on the domain in application. The trick is finding the balance between end-user configurability of layers n+1 and the required knowledge to design useful structures. People who edit layer 2 should probably be domain experts, and layers 3 and above are best left to programmers and computer scientists. If this was a solved problem, nobody would buy CRUD-software, and a good half of us would be looking for work.