since "e" is a smooth divisor, its normal bundle is a line bundle. it is not difficult to show that "e" intersects itself negatively. this means that its normal bundle possesses no holomorphic sections; "e" is the only smooth complex representative of its homology class in formula_40. (suppose "e" could be perturbed off itself to another complex submanifold in the same class. then the two submanifolds would intersect positively — as complex submanifolds always do — contradicting the negative self-intersection of "e".) this is why the divisor is called exceptional.