Jeffrey Donaldson gave little away as he arrived to board a flight from Belfast City Airport to London.

Asked by a BBC reporter yesterday morning about the prospects of a protocol deal and whether his Democratic Unionist Party might back it, he replied that he was "neither positive or negative".

He added: "We need to take time to look at the deal, what's available and how it matches our seven tests."

For a political party born out of religious fundamentalism, it was perhaps no coincidence that the DUP set seven tests.

It is a number with huge biblical significance. In scripture, seven is the number of perfection.

Jeffrey Donaldson is too shrewd a political operator to think that he can achieve all that his party is seeking and that political deals require compromise on all sides.

The question will be how far short of perfection will the DUP leader and his senior advisers accept?

So what are those seven tests and how might they be met?

The party kept them fairly general. For example, while in recent weeks senior party members have insisted that any jurisdiction for the European Court of Justice in Northern Ireland must end as part of any deal, the ECJ is not actually mentioned by name in the seven tests.

The DUP's flexible nature could work both ways: it could make it easier to say its tests have been met, or leave so much wriggle room the party could conclude that a number have not been addressed.

The party says any new arrangement between the UK and EU must fulfil these seven requirements:

1. Fulfil Article 6 of the Act of Union 1800

This is a constitutional statute that created the UK. Jeffrey Donaldson has said addressing this is essential as the article "requires that everyone in the United Kingdom is entitled to the same privileges, and be on the same footing as to goods in either country, and in respect of trade in the United Kingdom."

The DUP argues that the Northern Ireland Protocol is a breach of this article as Northern Ireland has different trading arrangements than England, Scotland and Wales.

2. Avoid any diversion of trade

The DUP says it would not be acceptable for consumers and businesses to be told they must purchase certain goods from the EU rather than from Britain.

3. Not constitute a border in the Irish Sea

The party says the fact that there are checks on goods coming into Northern Ireland from Britain, that do not take place when the same goods move within Britain, means Northern Ireland is subject to an internal customs border that undermines its constitutional position within the UK.

It is possible these three issues could be addressed by the introduction of green and red lanes for goods arriving at ports in Northern Ireland.

Only goods that would then cross the border into the Republic, the European Single Market, would have to go through the red lane and be subject to checks.

The DUP will want full details on how the lanes would operate and be monitored and will likely demand an end to all checks that were not carried out before the UK left the EU.

4. Give the people of Northern Ireland a say in making the laws which govern them

Also referred to as "the democratic deficit", this is where the talk about an end to the jurisdiction of the European Court of Justice comes into play.

In recent weeks and months senior DUP members have said it would not be acceptable if trade in Northern Ireland remains subject to laws made in Brussels that now no longer apply in England, Scotland or Wales.

"It's quite simple," a senior DUP source told me.

"Forget all the detailed legal language. It comes down to whether a manufacturing company based in Northern Ireland will be bound by the same laws that apply in other parts of the United Kingdom, or whether they will be bound by laws make by Europe. The latter is simply not acceptable."

The EU has insisted it the ECJ must be the final arbiter in any disputes about the application of rules and standards, as it is in all member states.

Standing alongside British Prime Minister Rishi Sunak when the Windsor Framework was announced, European Commission President Ursula Von der Leyen made it clear that some European laws will have to remain applicable in Northern Ireland and that the ECJ will be the final arbiter in any dispute.

The framework proposes a potential mechanism to overcome this - the so-called "Stormont Brake" - which Rishi Sunak said would enable the Stormont Assembly to block the introduction of any new EU laws.

But the process is complex and could only be used "under the most exceptional circumstances and as a matter of last resort."

There is also confusion about exactly what is meant. The Windsor Framework states that the "emergency handbrake" would be applied if 30 or more members of the Stormont Assembly from at least two parties sign a petition of concern.

Under the terms of the New Decade, New Approach agreement which formed the basis to restore power-sharing in January 2020, following a 14-day consultation period that petition of concern would then be put to a cross-community vote and would require a majority of both unionists and nationalists support it.

However, DUP sources have suggested they do not believe the need for cross community would apply and that the petition of concern itself would be sufficient to exercise a "veto."

That is an issue the DUP and other parties are urgently seeking to have clarified.

Another question is how would such a veto fit with Ursula Von der Leyen's insistence that the European Court of Justice will have the final say? Does that mean the ECJ could effectively ignore the veto?

5. Result in "no checks on goods going from Northern Ireland to Britain or Britain to Northern Ireland (and remaining in Northern Ireland)

As with tests 1, 2 and 3 this may be addressed by the red and green lanes.

6. Ensure no new regulatory borders develop between Northern Ireland and the rest of the UK - unless agreed in Stormont

This is where the argument over the jurisdiction of the European Court of Justice again comes into play.

The DUP has said businesses and goods in Northern Ireland must be subject to the same laws and regulations as the rest of the UK, so the introduction of any new laws from Brussels would create a new "regulatory border" within the UK.

7. Preserve "letter and spirit" of Northern Ireland’s position set out in the Good Friday Agreement

This test states that the Good Friday Agreement requires the consent of a majority of people in Northern Ireland for any reduction in its status as part of the UK.

In a statement last night, Jeffrey Donaldson welcomed "significant progress" on a range of issues, but stressed that there remain "key issues of concern", with the continued application of EU law in Northern Ireland one of them.

The DUP leader said his party will take its time to study the detail of what has been published and the underpinning legal texts before delivering its final verdict and deciding whether this agreement is sufficient to it to end its boycott of power-sharing.

In an ominous note for the British Prime Minister, he added that the party stands "ready to engage with the Government for further clarification, re-working or change as required."

That suggests the DUP may not think the negotiating is over just yet and that its search for perfection may not be at an end.