Right, concentrate. Focus. Brexit amendment time.

The Prime Minister will whip Tory MPs to vote for the Brady amendment tomorrow. At the 1922 committee meeting of Tory MPs this evening May asked MPs to endorse the chairman of the ’22’s effort at compromise. If passed it would commit the government to seek “alternative arrangements” (that Number 10 cannot yet explain) to replace the Irish border backstop that the EU says is non-negotiable.

On that basis, the government will whip for the Brady amendment. The hardline Tory Brexiteer group, the ERG, will vote against.

Confused? Even the people most closely involved in this are now confused about who is voting against what and why.

So here’s a little explanation on the context ahead of tomorrow’s key votes on Brexit. The Speaker will select various amendments to be voted on which could reshape the course of Brexit or oblige the government to alter its policy.

There is the Brady amendment – more of that in a moment. But, first consider the cross-party Cooper amendment. It is named after the late great comedian Tommy Cooper. No, just checking you are still with us. Obviously, named after former Labour cabinet minister Yvette Cooper MP.

“For parliament to ‘take control’ of the Brexit process would be unconstitutional,” the constitutional historian (and one-time tutor of David Cameron at Oxford) Vernon Bogdanor wrote at the weekend. He was referring to Cooper’s ploy. It provides that, if parliament fails to coalesce around a deal by February 26th, Brexit should be delayed by nine months.

In the UK, we have no written constitution, but we do have a whole paraphernalia of generally accepted practices and legislative checks that effectively do the heavy lifting in our constitutional arrangements. Bogdanor points out that for Cooper’s amendment to function it would need to suspend standing order number 14 of the House of Commons, introduced in the Attlee era, that gives priority to government business.

Extending Article 50 if the EU agreed also has public spending implications (future EU budget contributions for one thing), which can only be authorised by the executive, not on the whim of backbenchers.

Tomorrow, the argument will be tested. But it is not guaranteed to pass. Labour’s front bench team is divided on it, supportive in principle but unsure about the wisdom of officially whipping a vote on an issue (delaying leaving the EU on the timetable set out two years ago) that many opposition MPs have, shall we say, strong views on.

Labour leader Jeremy Corbyn is also worried about what the voters will think. John Trickett, shadow Cabinet Office minister, told the Guardian that the move was tantamount to “ignoring the views of millions of ordinary folk”.

So much for the Cooper amendment.

Potentially even more significant in many ways is the Brady amendment, brought by Sir Graham Brady, chair of the backbench 1922 committee, which calls for Parliament to require the backstop be replaced with “alternative arrangements to avoid a hard border” as part of the withdrawal agreement.

That is a compromise form of words, designed to allow Brexiteers to back down and support May’s withdrawal agreement in return for the EU and UK government perhaps patching on to the agreement some manner of extra reassurance about the undesirability of the backstop.

It has not worked with the hardline pro-Brexiteers in the ERG. Despite May’s plea they have signalled this evening that they won’t be voting for any amendments. Bernard Jenkin MP told ITV that he wouldn’t be voting for it as it now stands, which rather supports the orthodoxy in Brussels that the Brexiteer ultras in the Tory party will not vote for the deal in any form, which makes compromise less desirable from an EU perspective.

Look at EU deputy chief negotiator Sabine Weyand’s comments today. She noted that the deal stood no chance in parliament as the vote went down to a “two thirds defeat in the commons… crushing by any standards” but also reiterated that there is no chance of renegotiation on the withdrawal agreement, affirming that “there continued to be full ownership on the EU side of what was agreed.”

An alternative explanation is that the Brexiteers were keen for May to put forward her own amendment, which certainly might have made more constitutional sense. Iain Duncan Smith told the BBC’s Laura Kuenssberg: “This is the time when the PM must make clear her intentions in the negotiation to seek support. We need more than back bench amendments with nods and winks from the government.”

He’s right in a way but the government has put forward its pitch and it was defeated by 230 votes – the largest defeat by a sitting administration in democratic history.

It is all a dreadful mess. With the Cooper measure up in the air, and the Brady compromise amendment backed by the government but opposed by the ERG, May will, again, need all the luck she can get tomorrow.