When the Prime Minster said she would ask the EU to reopen the Withdrawal Agreement and replace the backstop with alternative arrangements to keep open the border on the island of Ireland, she presumably knew this was the equivalent of asking the chief rabbi to have a bacon sandwich or the Pope to marry.
Theresa May must have been told by her officials that she could ask all she likes, but they could not conceive of circumstances in which she would get her way - not least because she would be asking to rip up a treaty that was at least as much hers as the EU’s.
And since I am sure they did give that counsel, they will be wallowing in a deep warm bath of schadenfreude.
According to well-placed sources in EU capitals, there is still zero chance of the backstop being reworded so that it has an end date or any mechanism for the UK to unilaterally withdraw from it.
And they are not exactly reluctant to make this clear, to anyone who asks - including May.
That means the majority of 16 she enjoyed in winning the vote on the Brady amendment says absolutely zilch about whether she could win a majority should she ever have the gumption to put a reworked Brexit plan to MPs for ratification - because absent a reworked Withdrawal Agreement, around 100 Brexiter MPs would once more vote against her.
So the presumption has to be that May made the pledge to the ERG Brexiters of trying to amend the Withdrawal Agreement largely to buy a few days thinking space in the Micawberish hope that something else will turn up that would miraculously allow a Brexit plan with her moniker on it to be ratified by MPs.
All of this is not to say that there is nothing for May to negotiate as and when she speaks again with her counterparts in Brussels, Paris and Berlin. It is just that the negotiation will not and cannot be about the Withdrawal Agreement.
Amazingly - and with just eight weeks left before Brexit Day - May has still not told anyone in the EU what she might ask for, when she might ask and who she might ask.
But I am reliably told that the only document she could renegotiate with the European Union is the short political declaration that sets the framework for the UK's longer term relationship with the EU - and this could include, for example, a detailed scheme for how the backstop could be made redundant by technologies for verifying in a frictionless way, and possibly away from the border with the Republic of Ireland, whether goods and food meet EU standards.
This would be a confidence building exercise to reassure everyone that if the backstop was ever implemented, it could be made redundant very fast. However it would not be a legally binding commitment that the backstop would not last longer than the UK deemed desirable.
So it would reassure pragmatic Tory MPs but not the Brexiter ultras.
All of which simply means,as I said on Thursday, that the PM cannot surely - for much longer - dodge the most difficult choice of her life: commit to a version of Brexit that turns the backstop into a bridge to a permanent destination of UK membership of the customs union, to win possible support for her Brexit plan from Labour MPs, but in the process alienate perhaps a third of her own MPs, or stick with the Brexiter ultras and risk a no-deal Brexit.
It is a choice between a no-deal Brexit that might (only might) keep much of her party intact but would (by her admission) seriously damage the UK's prosperity and security, or a negotiated customs-union based deal that might be less disruptive for the UK but could dismantle her party.
All her words and deeds suggest that she is confident she will never have to make this choice, that the EU will blink and swerve as the Brexit train picks up momentum towards that no-deal Brexit (which they also see as a bleak destination, if not as desolate as for the UK).
There is no sign of the EU panicking.
If there were to be a bit of blinking in the capitals of the EU 27, I am told it could be to offer the UK an extension of two or three months before we Brexit, to ensure every last avenue has been explored for securing ratification of an unchanged and unchanging Withdrawal Agreement.
But it is for May to give them confidence there is anything constructive left worth discussing. And they say she has furnished them with no comfort.