A CELEBRATED Punch cartoon from just before the second world war shows a military man sprinting from a government building to a taxi, crying urgently: "To the Royal School of Needlework, and drive like hell!"

Similarly incongruous scenes may soon be witnessed in newsrooms across Britain. On May 5th and 6th, results will break from a referendum on whether to change the voting rules for British general elections from first-past-the-post (FPTP) to the Alternative Vote (AV). At that point, hard-boiled news editors will be heard barking at quaking reporters: "Get me a constitutional historian on the line, I don't care what it takes."

If those news reporters know what they are doing, they will try to get hold of Professor Vernon Bogdanor, who recently retired after four decades plumbing the mysteries of the British constitution at Oxford (where David Cameron was a star pupil). On March 31st, the professor published a slim volume examining the constitutional implications of the coalition government headed by his ex-pupil, and of the reforms being planned by that government.

I have reviewed the book in this week's print issue, and though steeped in learning, it is not remotely the product of an ivory tower. Mr Bogdanor puts his scholarship to shrewd, worldly use, explaining how much conventional wisdom about Britain's constitutional pipework is simply wrong.

He is especially interesting on how two of the most common arguments about AV are wrong. As I write in the review:

...it is a commonplace among the political classes that AV matters because it would hand the Lib Dems perhaps 20 more seats than FPTP, a system that brutally disfavours smaller parties

Not so fast, says the professor. Firstly, FPTP actually punishes parties with diffuse geographical support (just ask the Tories, who won one in six Scottish votes at the last general election but picked up just one in 59 of Scotland's seats, Labour in southern England and Lib Dems everywhere)

Secondly, he argues, the version of AV being proposed in Britain may not actually change very much at all.

The only large country to use AV for national elections, Australia, forces voters to rank all candidates, he notes.

Bagehot confesses to considerable relief that that is not the version of AV being offered in Britain. Australian readers should feel free to correct me here, but my hunch is that most British voters would deeply resent being told they had to assign a preference, even if it were only a fourth or fifth preference, to parties they dislike or even loathe, or have their ballot paper count as spoiled.

But instead, the version being offered in Britain will allow voters to write in a first preference, and leave all others blank: the professor calls this practice "plumping."

This is very significant, Mr Bogdanor argues, and he has the data to back this up. He notes that the stated purpose of AV is to avoid the anomaly by which a candidate can win a constituency on a minority of the vote. In the days when the Liberals and other small parties only contested a handful of seats, MPs elected by a minority were very rare: in 1951, for example, just 39 MPs out of 625 were elected on a minority vote. In recent times, however, seats contested by just the Conservatives and Labour have vanished. In 2010, there were an average of seven candidates in every constituency, and 433 out of 650 seats saw MPs elected by a minority of votes cast in their seats.

However, he explains, it is not correct to say that AV ensures every MP is elected by a majority. In the Australian states of Queensland and New South Wales, "plumping" is allowed in elections to state legislatures. And where it is allowed, it is very common. He records:

The greater the degree of plumping, the more an alternative vote election turns into a first past the post election...In Queensland, in 2009, where the Labor Party advised its supporters to "Just Vote 1", to give Labor their first preference and not to give a preference to any other candidate, around 63% of voters plumped. Even where a party does offer advice, that advice may be ignored. In Queensland, the Greens advised that second preferences be given to Labor, but 46% of Green voters decided to plump

He offers further examples from South Australia and the western states of Canada in the first half of the twentieth century, where plumping reached a peak of 68% in Manitoba in 1945. Such plumping means it is perfectly possible for MPs to be elected by minorities.

There are many arguments for and against AV. Many will be rehearsed here over the next few weeks (you lucky people). But for now, consider this possibility: by avoiding a dreadful form of AV (one which would make the use of all preferences obligatory), British backers of AV may have chosen a system that amounts to a gussied-up form of FPTP with added complexity and aggravation.