The desiderata were not 'writing a quick directory traversal function'; these were and are a dime a dozen. The writer explicitly demanded something you could o

The desiderata were not 'writing a quick directory traversal function'; these were and are a dime a dozen. The writer explicitly demanded something you could only contemplate in the context of a few programming languages, namely an unfold - fold over the directory system. This is why he rejects all of the no-brainer solutions; he repeats this again and again.