I've been asking officials and ministers for the prime minister's cunning plan to solve the seemingly impossible Brexit puzzle - of proving to her Brexiters that the Northern Ireland backstop plan would be temporary while avoiding any specified fixed termination date (because a backstop with a fixed termination date cannot, by definition, be a backstop; to mix metaphors, it would be a cliff edge).
Here is what I've been told: “The backstop cannot be limited by a fixed date” said a member of the government (telling you and me what the EU insists upon, but what Tory Brexiter MPs see as heresy). “But it might be capable of limitation by reference to a formula or a test to establish redundancy”.
Hmmm. A formula. Intriguing.
It all sounds plausible but establishing a credible test that would prove the backstop has outlived its utility will not easy.
Just to remind you, the backstop is the sine qua non of a Brexit Withdrawal Agreement because both the UK and the EU have agreed that this country's departure from the EU must not transform the current soft, permeable, open border between Northern Ireland and the Republic into a stickier or crunchier border (because of the risk that border checks would undermine the young foundations of peace in the region).
So the UK and the EU have coalesced around a backstop or insurance policy that would see Northern Ireland remaining in most of the EU's single market and the whole UK staying in an EU customs area.
If this backstop were in force, there would be no need for border checks between NI and the ROI. But equally it would prevent the UK from agreeing free trade deals.
Which is why Theresa May insists the backstop can only exist for as long as it takes the UK to negotiate and implement the detail of a new sui generis trading and customs arrangement with the EU.
But even though the backstop is temporary by definition, it gives Brexiters the willies; they fear the EU will never finalise negotiations on a special new trading and customs deal with the UK, either because the EU will say such a deal wouldn't really keep open the Northern Ireland border or the EU will see it as somehow flouting the important theology of the EU's single market.
Which is why May and her team are trying to devise an objective test of whether the backstop had been superseded by her Chequers plan (or any other proposal for the UK's long-term future trading relationship with the EU that may ultimately be agreed).
Now it seems to me that the only way such a test could be impartial would be if the test were administered by neither the UK nor the EU but by some neutral third party.
So it will fly, with Brussels and Tory Brexiters, only if they respect and have confidence in that third party.
However what I mostly take from all of this is that May is gradually working up the courage to face down the true Brexiters in her party - because she is poised to concede what they hate, and what the EU has insisted on, namely that the backstop should end only when it is practical to do so, and should not be subject to any kind of artificial term limit.
Brace yourself for the inevitable cries of traitor from her parliamentary colleagues.