There is no actual agreement between the British and Irish governments and Mr David Trimble as to what would constitute a "default" on decommissioning, forcing the suspension of the Northern Ireland Executive.
This has become clear with just four days to go to the expiry of the Ulster Unionist leader's deadline for the start of IRA disarmament.
Sources close to Mr Trimble last night confirmed his determination that Gen de Chastelain must report an actual start to IRA decommissioning by next Monday, and that failure to do so should see Mr Peter Mandelson reinstate direct rule sometime before the Ulster Unionist Council reconvenes on February 12th. Mr Trimble's apparent failsafe is his post-dated letter of resignation as First Minister.
The Taoiseach, Mr Ahern, has said he understands why Mr Trimble feels it necessary to stipulate this deadline in order to secure the UUC's backing for his decision last November to proceed into government with Sinn Fein. Similarly Mr Mandelson, when pressed in the Commons to persuade Mr Trimble to shift his deadline, said it was not of itself a bad thing to have the agreement on some form of "probation". However, neither Mr Ahern nor Mr Mandelson has actually endorsed Mr Trimble's interpretation of the "understanding" reached at the conclusion of the Mitchell review.
In his interview in this newspaper last Friday Mr Mandelson said he "broadly" agreed with Mr Trimble. While he wouldn't call it "an understanding", as such, the Secretary of State said he thought it was "understood" by the parties that if the unionists moved first on devolution, then decommissioning would follow.
However, there is a world of difference between that and Mr Trimble's assertion that it must have followed by January 31st. Mr Mandelson made it plain he was not getting into the business of setting deadlines.
And authoritative sources have told The Irish Times that, as far as ministers are concerned, the only operative deadline is that envisaged in the Good Friday accord - May 2000; that the quality of Gen de Chastelain's report cannot be solely a matter for adjudication by the Ulster Unionists; and that a "progress" report could amount to something less than an actual start to the surrender or destruction of IRA weapons by Monday.
Specifically, they say that "default" would clearly arise were the general to report that there was no evident commitment to decommission; that the appointed interlocutors lacked the authority to advance the issue; and that there was no serious consideration of the modalities for decommissioning, resulting in a failure to identify a timetable by which it might be achieved.
If the IRA representatives obliged Mr Trimble by informing Gen de Chastelain that they have no intention of decommissioning either side of May, the Secretary of State might think it an open-and-shut case. But that is not the republican way. Even those commentators most convinced that the IRA will not decommission anticipate no shortage of language with which the general might accentuate the positive.
Determined not to let republicans off the hook, British sources say "warm words" alone will not suffice, and that Mr Mandelson will be "rigorous" in his assessment of the quality of Gen de Chastelain's report.
In the Commons on Tuesday night his Junior Minister, Mr George Howarth, signalled a possible approach, suggesting Mr Mandelson's decision might be governed by an assessment of republican intentions. "The Secretary of State will have to be satisfied that there is the intention to make progress and that there will be a programme that will lead . . . and that there will be progress toward the commitment that exists within the Good Friday agreement," Mr Howarth said.
It is certainly conceivable that Mr Trimble would settle for a clear-cut timetable guaranteeing total IRA decommissioning by May 2000. However, the feeling in the Trimble camp is that if the IRA is prepared to give such an explicit commitment, there would be no advantage in further delaying commencement. And while they might wish it, in truth most members of the British and Irish establishments would be astonished by any republican promise delivered in such definitive or time-specific terms.
Officials are telling a simple truth when they say Mr Mandelson cannot know his options until he has the content of the general's report. And there is no doubt either that Mr Mandelson is sincere when he says he would not wish to see Mr Trimble carry the burden or stand alone.
That said, the Secretary of State and the First Minister may well be set on a fresh collision course. For if Mr Mandelson (and Mr Ahern) do not consider the January 31st deadline born of agreement reached in the Mitchell review - and if they decide that, short of an actual start to decommissioning, the de Chastelain report constitutes real progress toward its ultimate achievement - would they feel justified in suspending the Executive, in face of Sinn Fein's stern opposition, at Mr Trimble's behest?
There is no doubt Mr Mandelson would consider almost anything in order to avoid the truly appalling prospect of closing down the new dispensation. While praying for a result for Mr Trimble, he must be hoping that the general will at least provide the Ulster Unionist leader with some "wriggle room" - and that Mr Trimble might choose to avail of it.
But will he? Could he, even if so disposed? Senior unionists believe that this time it is Mr Trimble who holds the ace card, and that, if he plays it, Mr Mandelson will be forced to suspend the Executive in order to avert a Trimble resignation from which there would be no way back.
All, as they say, is speculation. And it could, of course, be rendered redundant should the IRA decide first to organise that long-awaited "event" atop the Cavehill.