Something very unusual happened towards the end of the TV debate between the five main opposition party leaders: I learned something I didn't already know.
Perhaps I haven't been paying close enough attention, but when Ed Miliband and the SNP leader Nicola Sturgeon were clashing noisily over hypothetical post-election cooperation between their two parties, I suddenly realised: the SNP have no cards to play.
The Tory campaign posters show a miniature Miliband in Alex Salmond's jacket pocket: the idea is to convince voters that a future Labour government without a clear parliamentary majority would be beholden to a party that wants to tear the Union asunder.
After the debate exchanges, however, it seems abundantly clear that the SNP would have little or no power to exert their will over Labour - for the simple reason that, as Nicola Sturgeon spelt out in no uncertain terms, their priority is, and will remain, to keep the Tories out of power.
In 1979, as Ed Miliband reminded her, the SNP withdrew their support from a minority Labour government led by James Callaghan, the government fell, and at the ensuing election, the Tories under Margaret Thatcher were swept into Downing Street. The rest is history.
So picture the scene: it's the first Balls budget, and the SNP don't like his version of what they call "austerity-lite". If they refuse to vote for it, it won't be passed. So Labour threaten a vote of confidence, and the choice is clear: either the Scot Nats vote against the government, and risk the possibility of the Tories being asked to form a new administration, or they buckle, and either abstain or reluctantly troop into the Aye lobby with Labour.
Nicola Sturgeon may be a star debater, even if the snap post-debate poll suggested that Miliband outshone her, but she has given away her single strongest negotiating card. After what she said on that stage, it's all but impossible to imagine her instructing her MPs at Westminster to pull the rug from under a Labour government's feet.
That could be very good news for Mr Miliband, and it could be why he was so unwelcoming in the debate to her overtures. All the signs are that Labour will still lose a barrel-load of Scottish seats on 7 May, but in any post-election negotiations, if Labour end up with more seats than the Tories, he will be able to present the SNP with a clear choice: either you back me on key votes, or the Tories get another chance.
(What kind of deal the Tories might have to do with other smaller parties - UKIP, the Lib Dems, the DUP - is a whole different story: much may depend on who becomes Tory leader if, as expected, David Cameron stands down if he fails for the second time to win an overall Commons majority.)
It's a funny old election, though - here we are considering possible post-election parliamentary permutations with another three weeks still left until polling day. Unless something truly dramatic happens between now and 7 May, my strong suspicion is that the post-election period will prove to be a great deal more interesting than the campaign itself.
A few nights ago, I chaired a standing-room-only hustings event in the UK's second most marginal seat, Hampstead and Kilburn, held in 2010 by Labour's Glenda Jackson with a majority of just 42. (The most marginal seat is Fermanagh & South Tyrone, which Sinn Fein won five years ago with a majority of four.)
What struck me as interesting, in a constituency where by polling day there will have been 20 similar hustings events (I almost begin to feel sorry for the candidates), was the response to a question I put to the audience at the end of the evening.
"How many of you," I asked, "as a result of what you've heard tonight, have either changed your mind about how you intend to vote, or are thinking of changing your mind?"
I was expecting either no one to raise their hand - the conventional wisdom is that an exceedingly small number of people change their minds during an election campaign - or perhaps a handful. In fact, more than a dozen hands went up, and I went home wondering whether perhaps there is still time for some surprises.
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