The Sketch: So the IRA's days are numbered? So what's the number?

DIPLOMACY, ALMOST by definition, is what happens off-stage. Then, after sufficient rehearsals in private, a public re-enactment is presented, in which the principals say their lines and adopt the positions carefully blocked out over hours of unreported negotiation.

This is particularly so in Parliament, where the chance of an ad lib "a genuinely novel and unexpected departure from the expected dialogue" is small. I think we saw a fleeting glimpse of one yesterday, though you'd have to read the word "think" in a hesitant way to capture the extent of my uncertainty. It's possible I was overcome by the faint tension that hung over Tony Blair's statement about the talks on Northern Ireland, "the sort of grave solemnity which trembles on the edge of a giggle", not because you think what's being talked about is a laughing matter but because so much hangs on what is said.

Mr Trimble actually did chuckle at one point, taking the excuse of an over-heated compliment to the Prime Minister to get some of his tension off his chest. Tony Worthington had expressed the view that, if it worked, the Prime Minister's agreement last Friday would be "the most miraculous bit of negotiating ever achieved".

Mr Trimble's fixed expression gave way to a broad smile. Either he thinks it will be a miracle if it works or he felt the apotheosis of Mr Blair had now tipped over into absurdity (even Tony Benn offered Mr Blair his "sincere congratulations", leaving the Prime Minister momentarily lost for words).

The Conservatives had opted for a grumpy defence of Unionism, one that wouldn't let them off the hook entirely, but pointed out that the hook was very unfair and people really shouldn't blame them for wriggling. Mr Blair, on the other hand, must ensure that it is set deep. For understandable reasons he has shifted the burden of expectation on to Unionist shoulders. The only way they relieve themselves, he suggests, is to agree to the Executive going ahead under these terms, at which point he will make Sinn Fein and the IRA carry it for a while. But his principal argument is what you might call the Cliffhanger Rationale in which he concedes that we don't know whether Sinn Fein is acting in good faith or not, but points out that the only way to find out for sure is to call it on its promises.

For the Unionists to withdraw now, he implies, would be as churlishly antisocial as ripping the last 10 pages out of a library detective thriller. Surely they can't spoil the ending now? It isn't difficult to understand why the Unionists remain wary, when the terms of their guarantee remain so vague. "Should default occur," said Mr Blair, "the institutions are suspended automatically whilst we find a way forward." That "automatic" is a crucial word. Unionists are unlikely to feel happy if suspension of the Executive rests on the wishful interpretations of London or Dublin. But will the IRA's "unambiguous commitment" to decommissioning have to come within two days, or seven, or the month Mr Blair seemed to allow when he asked: "Is it not worth waiting 30 days to see if the undertakings made here are fulfilled?"

If a "few days" can mean 30 then might not "a few weeks" take us beyond the May 2000 date set for total decommissioning? The men in bowler hats ordered a plate of good Ulster sausage and suspect they've been given thinly sliced salami. One hopes they can be persuaded to eat, but that discussion will definitely take place in private.