Can a sealed arms dump be considered decommissioned? Is a concrete cap sufficient to do the job? Or does rendering the contents of such dumps permanently unusable require filling in with concrete? Would even that be enough?

Or might it be necessary to add some metal-destroying chemical so that - should some future generation of republicans decide to tear the weapons from their concrete case - they would, in any event, find them corroded and beyond repair?

These are likely to be among the practical questions exercising Tony Blair this afternoon as he and Bertie Ahern resume their leadership of the Weston Park summit.

Gerry Adams and Martin McGuinness might mock the musings of a journalist plainly suffering sensory deprivation from the "real news" blackout as the negotiation proceeds. However, if there really is the prospect of agreement - and if, as is claimed, destruction of weaponry is the British Government's "bottom line" - then these questions will inevitably form part of today's agenda.

Not by any means all of the agenda, for all David Trimble's insistence that there is only one issue to be addressed. But a central and inescapable part of it. Indeed, we know this must be so from Sinn Fein's own approach to the negotiations this week.

Attention is inevitably focused on Mr Adams's repeated assertion that the question of "all arms" will not be settled in compliance with the August 12th deadline, which is a consequence of Mr Trimble's original July 1st deadline for resignation as first minister.

Moreover, Sinn Fein affects (at least publicly) to consider the question of Mr Trimble's survival as Ulster Unionist leader as something of a sideshow. Not for nothing do republicans claim "owner ship" of the peace process, or calculate that the British government will not disengage from it at the behest of whoever occupies the unionist leader's office in Belfast's Glengall Street.

That said, Mr Adams also repeatedly asserts his confidence that the arms issue will be successfully dealt with. The Sinn Fein president is on record as saying there is a responsibility to ensure weapons do not fall into the hands of innocent children or less innocent "other elements".

What is in dispute between Sinn Fein and the British government, it seems, is not whether weapons need to be put beyond use, but the context in which the IRA might be expected to keep its promise to do so.

As we know, Mr Trimble agreed to rejoin the power-sharing Executive last May on the strength of that promise. And the immediate context for this week's crisis talks is seen as his resignation because the promise has not been kept.

Sinn Fein, however, does not recognise this context. They suggest the point about last May's Hillsborough negotiation was not really about Mr Trimble at all, but rather about the terms of a deal between the British government and the IRA, and the consequent commitment of both governments to the "full implementation" of the Belfast Agreement.

This will strike many people as utterly disingenuous. Certainly, Mr Blair and Mr Ahern thought the whole point about Hillsborough was to secure the terms for Mr Trimble's return, and thus, the survival of the institutions of government established under the aegis of the Good Friday accord.

Whatever about that, the present position is that republicans do not seek to retreat from the promise to put weapons permanently and verifiably beyond use. But they maintain it is for Mr Blair to "create the context" in which it will happen.

The British Prime Minister, it seems, is determined to test this theory to destruction over the next 24 hours. Sinn Fein, in turn, will require him to spell out in detail his projected route to "the full implementation" of the agreement.

Which brings us to the other central, inescapable item on the agenda - the whole policing, demilitarisation, criminal-justice, equality and human-rights agenda, concerning which republican and nationalist expectations re main unfulfilled.

How far Mr Blair will be able to go will not just be a matter of calculation about how much additional strain Mr Trimble's leader ship can bear. The British state has interests here of its own. Mr Blair will have regard to the advice of his security chiefs, and (though it is at present mute) his own domestic political audience.

Will Mr Blair break his alleged promise to Mr Trimble in order to honour his alleged promise to Mr Adams, and deliver new post-Mandelson policing legislation? If so, what will it contain?

The power of the Secretary of State (and his accountability to the still-sovereign British parliament) vested in a triumvirate of the Ombudswoman, the Oversight Commissioner and the Policing Board? Former prisoners in police uniforms?

District partnership boards empowered to buy in or contract out local policing services? Power of retrospective inquiry by a policing board responsible for the new Police Service into the activities of an RUC that will no longer exist? Up to the absolute limits of Patten, or beyond?

And never mind the relentless spotlight on Mr Adams. What of Seamus Mallon and the SDLP? How far are they prepared to push Mr Blair?

Mr Mallon has reportedly divined some "progress" on policing and demilitarisation over recent days, and will, doubtless, expect some more when the talks resume this afternoon.

But what if, at the end of the day, it remains insufficient for Sinn Fein? Will that, by definition, hold good also for the SDLP?

Without doubt, Mr Blair faces some extraordinarily difficult judgment calls, and Mr Trimble will feel some pain if a deal is to be done.

But the same might be said of Mr Mallon and Mr Adams. For how long does Mr Mallon think it might be credible for the SDLP to remain in ministerial office without embracing the new policing dispensation?

And, while no one would doubt Mr Adams' nerve, or his ability to hold-out indefinitely, can he calculate to remain immune from the consequences of choice?

Some Irish insiders believe that Sinn Fein and the republican movement are approaching a defining moment, whether they like it or not. People who opposed the focus on decommissioning as the sole litmus test of republican bona fides are impatient to see the republicans complete the journey to exclusively democratic means.

Even some who would still question the utility of an act of decommissioning want to see it happen now - for what it would say about Sinn Fein's approach to the entire political process. Is it to be a stable, evolutionary process or, as one source put it, a case of "the worse the better" scenario? Translated by Mr Blair and Mr Ahern, the question for Sinn Fein and all the parties this afternoon is stark. Are they sufficiently committed to the Belfast Agreement to make the necessary compromises to save it? Or are any of them prepared to risk the uncertainty and potential chaos of either suspension or fresh assembly elections?

Optimism may be misplaced, but it is hard to believe any of them would want to stand alone in answering "yes" to the second of these. Or that they might expect Mr Blair or Mr Ahern to provide them with any cover if they do.