What might a nuclear deal with Iran actually look like?
Was this the week when, at least in nuclear terms, Iran came in from the diplomatic cold?
The new political leadership in Tehran has agreed to an accelerated process of negotiation under which the long-running controversy over its nuclear programme is to be resolved, perhaps within a matter of months.
The shift in Iranian rhetoric is remarkable.
The new US-educated Iranian Foreign Minister, Javad Zarif, told a private meeting earlier this week in New York that Iran and the five permanent members of the UN Security Council had agreed to "jumpstart the process" with a view to agreeing first, as he put it, "the parameters of the end game, how we want to proceed with Iran's nuclear programme in a year's time".
Mr Zarif went on to suggest that the next step would be "to think about steps, starting with a first step that should be implemented in order to address the immediate concerns of two sides".
The goal, he asserted, was to move towards finalizing a deal "hopefully within a year's time".
So much for the proposed mechanics and the timescale, at least from the Iranian perspective. But what might a nuclear deal with Iran actually look like?
Matter of pride
Negotiators must resolve a complex mix of technical, political and in a sense emotional factors.
Iran has long argued that its national pride and sense of technical achievement must be understood - hence it is not simply going to abandon its enrichment programme.
Indeed it believes that it has every right to enrich as a signatory to the Nuclear Non-Proliferation Treaty.
Nuclear rights are all very well, but Iran also has obligations, not least to satisfy the persistent concerns of the UN-nuclear watchdog - the IAEA - not just about its past alleged military research programme, but also broader concerns about the current scale of its nuclear activities.
This in a nutshell is the problem. Most countries with nuclear power-generating reactors buy in their fuel from outside. Iran says it wants to take a different path, developing the full nuclear fuel cycle; the ability to mine uranium, process and enrich it, and ultimately to fabricate it into fuel for a reactor.
One level of enrichment gives you reactor fuel. But the self-same process if taken to a higher level gives you the building blocks for a bomb.
Iran's enrichment programme worries experts on three counts. Firstly there is its overall scale - far beyond what might be needed for a domestic civil power programme.
Secondly there is the fact that it has a small underground facility for enrichment that is too small for a civil programme but ideal, according to analysts, for a military effort.
Thirdly it is already enriching uranium by up to 20%, which is a much closer jumping off point to make bomb material.
The problem for the international community is not so much to prevent Iran from having a nuclear bomb. Iran insists that it does not want one.
It is to re-shape Iran's nuclear programme and to introduce a level of openness and verification that would resolve many of the ambiguities about the programme and to create a sense of confidence in its civil nuclear trajectory.
Iran's enrichment programme is not the only problem.
There is its development of a heavy water reactor at Arak which would give Iran a source of plutonium; an alternative route to a bomb. Then there are Iran's past nuclear activities which many believe had a military dimension.
Israel's Prime Minister Benjamin Netanyahu has long argued that Tehran must adopt a four-point plan if it is to address international concerns.
He says it must halt all uranium enrichment; remove all enriched uranium from the country; close the underground Fordo enrichment plant near Qom; and halt the development of its reactor at Arak.
This is the maximalist position shared both by Israel and many on the American right and it is clearly not going to happen.
Iran will not halt enrichment in its entirety.
But shaping a deal where there are constraints and limitations; where perhaps some facilities are closed; and where Iran signs up to a much more intrusive verification regime - the IAEA's "additional protocol" could well be elements of a potential bargain.
Of course there must be inducements too, and not just in the vital area of a progressive lifting of economic sanctions.
Iran is concerned about its position in the region and its wider relationship with the United States.
The fact that US Secretary of State John Kerry sat next to his Iranian counterpart in the meeting in New York is an important signal that bilateral business between Washington and Tehran must also be part of any deal.
It should be remembered that there are also a range of unilateral US sanctions against Iran which relate less to its nuclear programme but to its broader behaviour backing Hezbollah and the Syrian regime and what the Americans see as its support for terrorism.
A diplomatic "grand bargain" with Tehran may not be possible, but a nuclear deal will inevitably require a broader basis if it is to succeed.
Iran's key nuclear sites
Source: 1155/New Scientist Global Security