For the first time, the leaders of the 19-member euro area officially discussed plans for the departure of one of their members.

According to the draft proposals handed by the Eurogroup (the finance ministers) to their leaders for their overnight meeting, among the clauses to be debated was one worded as follows: "In case no agreement could be reached, Greece should be offered swift negotiations on a time-out from the euro area, with possible debt restructuring."

It is difficult to overstate the significance of this - for its entire life, the euro was conceived as a currency from which there could be no exit.

This was not accidental: the disasters which befell the Exchange Rate Mechanism in the early 1990s convinced European leaders that the only way to create a lasting single currency was never, ever to countenance anyone leaving it.

The euro was "irreversible", to use the word Mario Draghi has frequently used - but on Sunday night in Brussels it transpired that it is far from irreversible.

Euro finance ministers are now actively discussing giving Greece a "time-out" from the currency. 

Now, one should insert a major note of caution at this stage. The clause quoted above was not agreed by all the euro members here in Brussels - it was put into square brackets, meaning it is yet to be agreed by all member states.

It may well be excised by the time the leaders have honed the draft document away to produce their final statement.

Nonetheless, it was on the table.

And that means that to some extent, the genie is now out of the bottle. Brussels is officially discussing how to engineer Greece's departure - the euro is not irreversible. Clearly, they will not do "whatever it takes" to keep it together. 

Although the agreement which was eventually reached this morning will keep Greece in the eurozone if it is enacted - last night sets a precedent that expulsion from the currency is not off the table.  

Additional reporting from IRN