Here are the reasons why there must be and cannot be a general election.
First, the drivers of a general election.
1) Tomorrow MPs will start the process of identifying, via so-called indicative votes, a route through the Brexit mess that a majority of them can back.
2) This process is likely to continue next Monday, when a range of Brexit or no-Brexit options should be whittled down to one.
3) There will then be a vote, maybe the following day, compelling the prime minister to negotiate with Brussels whatever MPs have decided.
It is too early to say what option MPs will coalesce around. And maybe they are too fractious and divided to coalesce around any practical solution.
But my hunch is that they will go for either a referendum, or a softer version of Brexit than Theresa May's that would breach one or more of her red lines - for example, many MPs want the UK to be in the customs union, which would prohibit trade deals with countries outside the EU, and they favour the kind of costless access to the single market, which would make it impossible to take back control of immigration.
Here is the nightmare for Theresa May. If, as seems highly likely, MPs instruct the prime minister to negotiate a Brexit or no-Brexit outcome that conflicts with government policy, she and her ministers would be degraded into ciphers and puppets of MPs.
They would have no discretion any longer over the most important decision facing this country for generations, how and whether to leave the EU. That would feel to her like a constitutional abomination. And she might well be right.
At that juncture, surely, the prime minister would have to call a general election. Because absent a general election, she could never re-establish her authority. And with the genie out of the bottle that backbenchers not ministers are in charge, it would be impossible for the government to function normally, unless and until a new parliament is formed.
A general election simply cannot happen.
There are two big reasons why it cannot happen.
First is that the two main parties are irredeemably split over Brexit. Labour and the Tory Party would find it impossible to craft a manifesto with a policy on leaving or not-leaving the EU for which all their respective MPs could campaign.
So an election could not end the torture for the UK and EU of the uncertainty about our Brexit or no-Brexit future.
Second, I have not found a single Tory MP who think it would be a good idea for Theresa May to lead her party into another election, such is the degree to which they have lost confidence in her (and she of course has pledged not to do that).
Also it would be asking the EU to have patience that even Job lacked to request a Brexit postponment long enough for the Tories to first have a leadership election and then go to the country for a mandate.
So, as I said, the Letwin plan has set the UK on a path to the impossible general election.
There is a risk, a meaningful one, that a general election could be forced on us, against the wishes of the PM and cabinet - via vote of confidence in them that Tory ERG Brexiters and DUP would support if the alternative was a form of Brexit or no-Brexit they view as worse than the risk of Labour winning that election.
This is not alarmism. The DUP is not the Tory Party, and is motivated by its standing in Northern Ireland, not by how it is seen by Tory supporters in England. And there are dedicated ERG Brexiters who view the battle for the one true Brexit as trumping everything else.
Now an election called against the will of the Tory party, with Theresa May at the helm, would eviscerate that party.
Well the only get-out-of-jail card for the PM and Conservative Party is that some time between now and 11 April, Tory MPs, the DUP and even a smattering of Labour MPs are so alarmed by the looming political armageddon (I choose my words with care) that they at the last choose to back the PM's Brexit plan (I identify 11 April as the true deadline, because although the EU's 27 leaders have identified the end of this week as the deadline for her deal to pass, I understand they would cut her slack till then).
Truthfully I would wager little on the PM winning at the last. But I would wager absolutely nothing at all if Theresa May continues to resist the call from her colleagues to very publicly promise that she will stand down the moment her deal is ratified.
Her loyal colleagues in the Cabinet and No.10 (there are still a few) tell me she resists announcing her departure only out of pragmatism, not self-interest, that she is unpersuaded that falling on her sword will get the deal across the line.
She is right of course. Her ritual sacrifice may not be her victory at the last. But pretty much every minister tells me privately her deal cannot come back from the dead unless she pledges her own demise.
If the PM naming a date for her resignation is a necessary though not sufficient condition for the UK to leave the EU with a deal, and I can find very few Tories who believe otherwise, she'll be gone soon. Logic does occasionally apply to politics, even in this crazy Brexit era.