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Nicola Sturgeon says she has no plans, right now, to introduce a quarantine period for visitors coming into Scotland from other parts of the UK. But by saying she is also "not ruling anything out", the first minister has ignited another heated constitutional debate.

Ms Sturgeon says we are not dealing with politics at the moment, but with a virus that doesn't respect borders or boundaries.

"Anyone who is trotting out political or constitutional arguments is in the wrong place," she says.

But the first minister was obviously well aware that even mentioning such an idea would cause a huge political controversy.

The Scottish Tory leader, Jackson Carlaw, has said the virus "should not be used as an issue to drive a wedge between Scotland and England".

The suggestion of a quarantine was not the idea of impatient pro-independence campaigners, but was first raised by one of the most eminent public health experts in the country, Professor Devi Sridhar, who is also an advisor to the first minister.

As experts now predict that Scotland could be on course to eliminate (not eradicate) the virus by the end of the summer, questions are being asked about whether Boris Johnson is aiming at a policy of elimination for England - or if the UK government is content to allow Covid-19 to continue to circulate at a higher level as long as it does not threaten to overwhelm the NHS.

Scotland has been pursuing a quite separate policy toward lockdown easing for weeks now, with a much much slower and more cautious opening up of the economy.

So the idea of a quarantine at the Scottish border doesn't seem completely outlandish to people who are now very accustomed to seeing very different rules operating across all four UK nations.

If we can see local lockdowns being imposed on a city like Leicester then why not have rules about crossing the border into Scotland?

It's clear the Scottish government has given the idea some serious thought as Ms Sturgeon drew comparison with the US, where some states have quarantine regulations, and the travel restrictions currently in place in different parts of Germany.

In practice, such a policy it would be bound to throw up all sorts of anomalies.

What about people who live and work on different sides of the border? Or families spread throughout the UK who want to get together for a summer break?

And it would draw howls of protest from the hard-hit Scottish tourist sector, which needs to welcome visitors from the rest of the UK this summer if many businesses are to survive.

Not to mention the extremely contentious question of how it would be enforced.

Police patrolling the border? That really would trigger a political earthquake, even if it was being done in the name of public health enforcement.

A Scottish quarantine would be complex and it would be seen as very divisive, at a time when all politicians say the coronavirus crisis is far more important than any constitutional debate.

But that doesn't mean it is ridiculous or impossible.