Today may be the day when we discover if Brexit is even theoretically possible. One could characterise the rest of the debate and disagreement over three long years as little more than a fog obscuring the one ineluctable issue.
Put simply, there can be no customs infrastructure on the border between north and south in Ireland, because of the GFA.
There could in theory be a line drawn down the middle of the Irish Sea, but this is unlikely to be acceptable to the Unionists in Ulster, who will surely never agree to the territory being treated differently to the rest of the Union. (And nor frankly, should anyone else in it.)
So here we are; the fundamental choice between two unacceptable alternatives. They were always there, and this is why Cameron had no business promising a YES/NO referendum on EU membership without addressing all the treaty obligations first.
Legally, the options on the ballot should have been NO/NO. For all the rest of the contentions thrown up by this controversy: sovereignty, immigration, trade and so on are secondary. To some extent irrelevant. The UK needed to look long and hard at its own constitutional arrangements, before it could propose to opt out of the treaties that made it part of a 'United Kingdom' and of the EU 28.
Right now, given that all sides must know that this problem is not open to solution, especially given the artificial deadline, all they can do in order to at least appear to be trying to resolve it in good faith, is to come up with a fudge whereby the customs infrastructure and the 'invisible' line are shunted somewhere in between the Irish sea and the actual border. In practice that would be almost as unworkable as the original two non-options.
'No deal', now formally also illegal thanks to the Benn Act, was also no solution.
The only matter that can be properly clarified now is a political-historical one. Who gets the blame for this strategic mistake of epic proportions? (Other than David Cameron of course...)