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In politics, contingency planning is a double-edged sword. You need to do it, otherwise if the worst comes to the worst you will be left flailing around in a panic. It's why there are presumably all sorts of plans in place for what happens if Greece leaves the Euro, the currency itself fails, Michael Portillo enters the Big Brother house or if the earth is hit by a massive meteorite.
The trouble - David Cameron is discovering - is when someone asks you about these plans.