As I play with it, however, I’m also reminded of Scapple, an app developed by the Scrivener crew. It’s like a mind-mapping app, but primarily textual, and is surprisingly powerful.

However, unlike Tree, it doesn’t do folding.

There are mind-mapping apps that do do folding, however, and even do quite sophisticated things with text – one of the versions of XMind, I seem to remember.