Europe’s leaders will have the unique opportunity to commit two mistakes in a single week. On Monday, Europe’s leaders will decide on the future of Greece. On Thursday and Friday, they will meet again to discuss, among other things, the future governance of the eurozone. The latter is more important in the long run: a healthy eurozone may even withstand a Greek exit from the single currency and prosper. But a crippled eurozone would be no less crippled if Greece were to remain a member. A dual failure would be a disaster.