 Let "Y" be the zeros of formula_40 in formula_41, and let "V" be the blowup of "Y" at the origin.The exceptional locus of this blowup is isomorphic to formula_42, and can be blown down to formula_43 in two different ways, giving varieties formula_15 and formula_45.The natural birational map from formula_15 to formula_45 is the Atiyah flop.