Lest anyone be in doubt about the amount of work involved, here is more from that background press briefing by that same U.S. official:

And I’m not being evasive in the sense in that we are entirely focused on working as hard as we can, as intensely as we can, as robustly as we can. And as I said, my colleagues are still working. We will be working on the airplane home. We will be working over the weekend. We will be working all next week. We will be going to Europe for consultations. We will be then coming right back here to Vienna. Our days started very early; they go very late at night. In the U.S. Government we have I don’t know how many people who are working on this who will never see the inside of a meeting room in which some of us sit, but are working on technical documents, are working on solutions, are coming up with ways that we might approach this to assure ourselves that Iran will not obtain a nuclear weapon.

Lots more work is planned, with the talks due to reconvene July 2 in Vienna, and quite possibly just keep going, right up to the July 20th deadline, or perhaps way beyond.

Why all this work? According to this same U.S. official, in the same briefing linked above, “There are very, very difficult decisions to be taken here by Iran.” (Yet more work, apparently).

Here’s the question. If Tehran is genuinely willing to scrap its quest for nuclear weapons, what’s so very, very difficult?

Under the labor theory of diplomacy, all this work and difficulty might herald a fabulous deal in the making. In the real world, it’s looking ever more like U.S. diplomacy is digging itself into a big hole. The advice in such situations: don’t work harder. Stop digging.