EU summit: UK deal in reach, but plenty could go wrong
Nothing is agreed, runs the diplomatic mantra, until everything is agreed.
And so it will prove at this summit.
A deal for the UK is within reach, but just one among a number of unresolved issues could delay everything.
Only when the 28 national leaders sit down to negotiate face-to-face will the extent of potential problems become clear.
Anything from proposed restrictions on child benefit payments to the meaning of the term "ever closer union" could provoke lengthy debate - lawyers will be on hand to go through the fine detail.
"It seems rather absurd," said one senior EU diplomat, "that prime ministers and presidents may have to spend hours debating the nature of restrictions on child benefit payments.
"But we are where we are."
The UK will fight back against a demand from Eastern Europe that changes to child benefit rules in the UK should only apply to new applicants.
Other countries are sensitive to the suggestion that changes to benefit payments, designed specifically with the UK in mind, could be adopted by other member states in the future.
In Romania, for example, with about two million citizens living in Italy and Spain, that is a big concern.
Part of the problem is that most countries didn't see a draft of this agreement until a couple of weeks ago.
There is a huge amount to discuss, particularly because it involves so many complex legal issues.
The UK demand for a cast-iron guarantee that the agreement negotiated by David Cameron will eventually be enshrined in the EU treaties is one of many challenging issues.
Is it a red line?
"I would say it is essential," said an EU source.
Financial regulation, the status of the City of London, and the language used to describe the relationship between countries inside and outside the eurozone, could also provoke a few late-night headaches.
All in all, there is plenty that could still go wrong.
But there's also a feeling that further delay won't make it any easier to reach a deal that everyone can live with.
"The intention of [the European Council President] Donald Tusk is very clear," one senior EU official said. "He believes this week is the best time to reach a deal on a new settlement for the UK."
"There is a mood around town," said another source, "that if we can't solve it now we're never going to solve it."
That is partly because the bar has been set relatively low.
It is hard to describe the draft agreement that has emerged as a fundamental renegotiation of the UK's relationship with the rest of the European Union.
Many of the demands David Cameron has raised in speeches and manifestos over the last few years have been watered down.
But there is real change in there, perhaps more than many people thought he could achieve just a few months ago.
And if, as appears most likely, a deal is done this week, the starting gun will soon be fired for a UK referendum campaign, which will define the political future of the prime minister and of his country.