It's a tree of lists of trees. I rather a tree, than tree-like :-) Some of our apps have deeply nested paths (typically a no-no, but actually quite appropriate

It's a tree of lists of trees. I rather a tree, than tree-like :-) Some of our apps have deeply nested paths (typically a no-no, but actually quite appropriate for our problem domain), so it is impractical to create a module and call "include" for each tree node. As for callable classes: someone has to instantiate them. That's no different than our custom routing and authentication. I just choose function composition over class inheritence.