Sorry, but can someone explain…if there is default, does that mean a literal "Grexit", both from the Eurozone and the EU itself the next day? Or will some kind of legal fiction be created for an interim period, to smooth things over? For example, with strict capital controls, could the Euros in Greek banks still be considered "Euros" in some fake legal sense, while TPTB figure out what to do, even though it's understood by all that that "Greek Euros" are treated differently from "real Euros"?
Or are we really going to see Greece formally leave the Eurozone (and the EU?) as early as this week? 
Here's what we know according to a economist I asked about this.
The way Greece will leave the Euro are that ECB will stop transfer Euro to Greece. Now the Greek government will need to either use it own saving of Euro (which it doesn't have), borrow from the banks (whose surplus of Euro have been emptied), sell something (not very likely) or loan money from foreign sources (which won't loan money to Greece). This mean that the Greek government need to print IOU to its domestic creditors (pensionist public servant etc.). These IOU will be the new currency, and by printing them Greece have de facto left the Euro zone.
Everything else is speculations.
But I guess that Euro in Greek banks will become the new currency, the rest of EU can in theory throw Greece out, but I doubt they will do it, Greece are not going to leave EU voluntary. But we may see Greek membership in EU being frozen. Meaning that they lose their voting right in the council of ministers, border control to Greece being reestablished and free movement to the rest of EU being frozen too. At until a deal for Greece future membership in EU have been reached.
There will be a lot of pain for a little while. The obvious thing to do if they leave the euro is to have a bit of a loose money policy to jumpstart the economy. Paying domestic creditors is easy, just create some more currency. Obviously inflation will be high, but inflation tends to be correlated with economic growth.
