'Grexit' - how does a country leave the euro?

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Amid political deadlock, the chances of Greece leaving the eurozone are growing by the day. For a long time this was unthinkable. But European leaders are now talking openly about the possibility of a "Grexit". For example, the governor of the Central Bank of Belgium talked about an "amicable divorce" (but would it really be amicable?)

Arguably, Greece should never have been in the euro in the first place. It's only 2% of eurozone GDP, so what ramifications would its departure from the currency bloc have?