Agreed. You can write prose or implementations or tests beforehand and I don't think it matters too much which you choose, just as long as you give yourself a phase 2 for incorporating the learning you did in phase 1 and put a reality check of some kind in between them.
The only problem with having the draft being implementation is that maybe you'll get pressured into shipping the draft.
The only problem with having the draft being implementation is that maybe you'll get pressured into shipping the draft.