The magician doesn’t want you to see what he’s really doing.  So he draws your attention to something minor, in the hope that you won’t spot what actually makes the trick work.

The Prime Minister’s ‘Dear Donald’ letter isn’t quite as sneaky, but there is more than a hint of the same ploy.

The migrant workers’ access to working benefits looks like it is set up to be the distraction.  It must be, because the Prime Minister went out of his way to stress how important it is!

It is the only part of the entire letter that offers a specific target to be achieved – the four year delay before such benefits can be accessed.  In a letter that is otherwise about broad objectives, why include so precise a target?  Why hold yourself up to potential charges of a climb-down if, as a result of the negotiation, you have to settle for something less than 4 years?  Why encourage so many followers of the forthcoming negotiations and subsequent campaign to conclude that it has to turn on the detailed negotiation surrounding an item that carries a maximum £500m annual cost to it – a trivial part of the total EU budget?

It has to be because this is where the Prime Minister would like the attention to be focussed.

And because he thinks the voters might actually care.

But, meanwhile, other things are happening, which are of much greater significance.

The ‘Dear Donald’ letter includes an itemised list of negotiation objectives in only one other area – namely currency arrangements.  This is where the core of the trick lies.

While all eyes are focussed on ’can he get 4 years or not?’, the Prime Minister – almost certainly working in cahoots with other EU leaders – is cementing into place a two-tier European economy and financial services regime.

He is seeking to construct an impregnable wall between the Eurozone and the rest of the member states.  He seeks to curb the ambition of the Euro project.  And in doing so, he may be undermining its foundations.

The ‘Dear Donald’ letter sets out a clear co-habitation arrangement for the Eurozone members and the non-members.  The arrangement is one of equality.  No longer is there an assumption that non-Eurozone members are just Eurozone members in waiting.  The Cameron vision is one that bolsters the position of the non-Eurozone currencies, and which draws clear lines of responsibility.

For one thing, it seeks to enshrine that no future Eurozone crises will in any way involve the non-members in giving any support to the euro.  Further, the Eurozone participants are curbed in their ability to make decisions which would impact on the non-participants.

Then there is the very interestingly worded non-discrimination clause – no discrimination against any business on the basis of currency.

This section of the letter concludes with an important statement of principle on financial sector regulation, where the equal status of non-Eurozone regulators is asserted alongside that of the ECB.  This can only mean that Mr Cameron does not anticipate a single financial services regulatory system devised within the Eurozone as having applicability beyond the zone.  If so, this implies an end to a concept of EU-wide financial services regulation as we have known it.

Much now depends on how these particular objectives emerge from the negotiation process.  But if they come out intact, the negotiations will almost certainly circumscribe the ambitions of the Eurozone, and make it more responsible for its own fate and less able to take decisions having any impact on those who are non-participants.

It will no longer be the currency arrangement that serves as the litmus test of EU integration

This will be the applications of the lessons from Greece.

And if it all comes to pass, there will be a muscular group of non-Euro currencies, led by sterling, forming a powerful counterweight to the Euro core – and with protected supervisory and regulatory authority.

If that comes to pass, in broadly the way Mr Cameron anticipates, might this rainbow European currency bloc eventually be seen as something of a lifeboat into which those who can no longer stomach the constraints of Euro membership can climb?

If so, it will mark the end of the Euro project as initially conceived.  Which will be historic.  And which could happen while all eyes are focussed on migrant workers’ benefits.