Things fall apart; the centre cannot hold.

It's more complicated than it looks, though, and here are three alternative options to throw into the speculation pot, each with a completely made-up percentage score for probability. 

1. Grand coalition. Labour and the Tories can get together, agree to do nothing on PR, agree some cuts but not Trident, and in the full knowledge that either party can trigger an election or realignment at any time. It worked better for Merkel, I understand, than her current deal with the right-wing liberal FDP. Falls down because Westminster is way more tribal than the Bundesdag. 8%

2. Labour minority. This would have required Gordon to stay on and bring a Queen's Speech which it would have been difficult for the Libs, the Nats & Caroline to vote down, including probably AV+ as a minimum. As it is, Brown can't do it, and there won't be a new (as opposed to New) Labour leader to deliver it in time either. 2%

3. Lib Dem split. This is the one which I now think is the most likely out of all the dark horse options. Dave will have 307 seats once Thirsk and Malton goes to the polls later in the month, and he needs just 323 for a bare majority given Sinn Fein's abstentionism. If you were him wouldn't you try to get through to some of the Orange Bookers and offer them some enticing options? Finding 16 of them would be hard, but a tacit or explicit deal with the DUP would mean the Tories would just need eight Liberals SDPers to switch. Invite Vince Cable, David Laws, Steve Webb and Ed Davey into Government and you're halfway there. They could call themselves the National Liberals and history would start to rhyme again, with a Grand Quasi-Libertarian Repeal Bill as the first item of business, followed by savage cuts. 10%

Going back to the most obvious options and sticking my finger in the air, I see a full Con-Lib coalition as about 20% likely, Lab-Lib working together with just the SDLP and the Alliance at about 15%, with some broader rainbow at about 10%. Within those options, Clegg as interim PM is unlikely but should not be completely discounted. A Tory minority is still at 10% despite Osborne apparently ruling it out.

A near-immediate second general election at 20% - this is the nuclear option which has to alarm all but the Tories, unless they can somehow be blamed for it. The remaining 5% is a figure, no doubt too low, for some option I've not considered.

As we found during coalition talks in 2007, if faced with no obvious options which work for you, look for something else. Change the question, change the game. Labour did their best on that front yesterday, leaving the Tories looking flatfooted and like yesterday's news. 

You have to hand it to Alastair Campbell: as he will no doubt have advised Brown, there was never a more important news cycle to seize, and no better way to do so than the PM's 5pm move yesterday. 

The Lib Dems are indeed in a horrible position, having got what they've long wanted. My advice to them, for what it's worth, is to use your imagination. A time-limited deal with a second election built in and Clegg as interim PM? Something that includes radical reform for a strengthened House of Lords? Who knows, but to every problem, even one as thorny as this, there is a best answer.