All: these guys did a Show HN yesterday at https://news.ycombinator.com/item?id=30615605 (there was a scheduling mixup on my part). I mention it here to (a) explain the dupe, for anyone who saw that thread; but also (b) to tell everyone that the discussion there was unusually high-quality, so you might want to check out those comments first.

Actually, maybe we should just merge that thread into this one. I'll double check if that makes sense.

Edit: ok, I've moved the comments in here now. Some of the times are messed up, but I think it makes more sense for the comments to be in one place so readers don't have to go back and forth. Sorry for any confusion!