And do you know why?
The answer appears to be the ERG and DUP think Cox can be persuaded to give advice that Article 62 of the Vienna convention will fly legally as a means of the UK unilaterally exiting the backstop. A62 basically says any nation can get out of a treaty by just quitting it if it thinks there is a fundamental change of circumstances.
That would allow the ERG and DUP to vote for May's deal.
It is however a ludicrous stance to take because as the deal has as a fundamental part of it that you can't unilaterally leave the backstop then if you have come up with a legal ruse that says you can unilaterally leave it, it is telling the EU and the world the UK can't be trusted.
It doesn't even matter if this is used as a ladder for the ERG and DUP to climb down and they themselves do not believe it would ever be used and this is just a face saving exercise as they have at the 11th hour reluctantly accepted the backstop. The fact they put it on the table leaves the possibility it could be used and so it can only be seen as a way to undermine the deal.
What the EU would make of this I have no idea but it would signal the UK was entering into this deal with every intention of reneging on the backstop as soon as possible having cooked up an excuse.
I think that would leave us seen as about as trustworthy as North Korea.