New York Times: Governments in Europe and elsewhere have begun rolling back generous, across-the-board subsidies for biofuels, acknowledging that the environmental benefits of these fuels have often been overstated. But as they aim to be more selective, these governments are discovering how difficult it can be to figure out whether a particular fuel – much less a particular batch of corn ethanol or rapeseed biodiesel – has been produced in an environmentally friendly manner. Biofuels vary greatly in …