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The Soviet Union said 40,000 workers toiled in ''exceptionally rugged terrain'' to finish the Siberia-to-Europe natural gas pipeline - a cause of strain between the US and its allies. The government newspaper Izvestia said the 2,766-mile pipeline - the world's longest - was completed in a little more than 13 months. Before the pipeline is operational, 40 compressor stations must be built to pump the gas.