REMARKABLY, we’re almost half-way through the passage of the SNP’s alternative Brexit Bill.

Such is the pace of the procedure underway at Holyrood that there has been little chance for MSPs, far less, outside experts, to immerse themselves in the detail. But a few early points have emerged about the Bill at the heart of a looming a constitutional crisis, and they are less than reassuring. It’s important but chewy stuff, so bear with me.

The SNP Government calls its EU (Legal Continuity) Bill a “backstop” measure in case it can’t reach a deal with the UK Government over the main Westminster Brexit legislation, the EU (Withdrawal) Bill.

As I write, the two sides are still unable to agree on how the withdrawal Bill should handle devolved powers being repatriated from Brussels at Brexit.

If they strike a deal, the continuity Bill will be junked. But if they can’t, the threat is that Holyrood will pass the Bill in 11 days’ time to transfer devolved EU law into Scots law so there is no legal cliff-edge at Brexit.

The UK will then face a political dilemma over whether to impose legislation on Holyrood, ask the Supreme Court to strike the Bill down, or concede power by amending its withdrawal Bill.

The Scots Bill is intended to mesh with the withdrawal Bill by taking care of devolved law and leaving the reserved powers to Westminster.

MSPs agreed to its “general principles” on Wednesday, with only the Tories set against it. But there are more votes to come, and a lot of questions about the fine detail.

Here are a very few examples. It is fiendishly complicated. “Complex, often difficult to interpret, and sometimes lacking in clarity,” as the Law Society of Scotland called the Bill. Many MSPs, like journalists, are wrestling with it. They are, to a degree, legislating in the dark.

Worse, they are legislating in the dark at high speed. The Bill is being put through as emergency legislation because it has to pass before March 22 to work. It has to pre-empt the UK withdrawal bill amending the 1998 Scotland Act in a way that would render it defunct. The two parliaments are in a race.

Holyrood’s eight previous emergency bills been narrow, single issue affairs, such as fixing a glitch with the Erskine Bridge Tolls, or dealing with a sick senior judge.

This Bill has 38 sections and three schedules over 34 pages, and relates to the Gordian knot of Brexit.

It is supposed to “dovetail” with the UK withdrawal Bill. But both are works in progress, facing amendments which could throw them out of synch. Holyrood is trying to legislate at a moving target.

At this week’s finance and constitution committee, Professor Aileen McHarg of Strathclyde University warned changes to the withdrawal bill could render the continuity bill “unworkable”. While Michael Clancy of the Law Society of Scotland said: “If one is a Scottish minister, one has to be be fleet of foot because, by the end of the process, the withdrawal bill could end up as quite a different measure from how it appears at the moment.”

Professor Alan Page of Dundee University added: “There are two different legislatures legislating and no guarantee that what comes out at the end will match up.”

The Scottish Bill, like the UK Bill, also gives an awful lot of power to ministers, and MSPs are kicking off.

The ‘Henry VIII’ powers in both let ministers to change most laws by decree for two year after Brexit to tidy up anomalies in EU-related law.

But the Scottish Bill, and its Welsh equivalent, have something extra. Section 13 of the Scottish Bill empowers ministers to to “keep pace with EU law” after Brexit.

Ministers would be able to shadow all EU regulations, directives, decisions and legislation in Scotland to “maintain regulatory alignment”. The government gives the example of mirroring updates in EU medicine and food additive rules to help ease of trade with Europe.

It sounds sensible enough. But there is concern that this power is going into the hands of ministers, not the parliament. MSPs will, at best, be able to yes or no to plans from on high, but not amend them.

Moreover, this power lasts five years, not two. And those five years can be extended for five years, and extended for five more after that, subject to parliamentary approval

As an example of how MSPs aren’t keeping up at the back, one SNP MSP told the chamber the power only lasted five years, while the LibDems said it was 15. In fact, it can be renewed indefinitely.

Prof Page called it “a potentially major surrender by the parliament of its legislative competence” and “a thoroughly bad idea”.

But perhaps the biggest problem, again identified by Prof Page, is that, despite talk of 111 devolved areas, the Bill does not say which powers coming back from Brussels are unambiguously reserved and which unambiguously devolved.

Instead, its definition is circular. It says devolved powers are those which are within Holyrood’s remit. But, tellingly, it doesn’t list them.

“That is simply to restate the question in another form, not to resolve it,” laments Prof Page.

It doesn’t mean the Bill is a train wreck, but it will test Holyrood’s capacity to the limit. The UK government may well wonder how much of a threat it really poses.