public static Set <T> except <T>(Set <T> setA, IEnumerable <T> setB) => setA.Except(setB);
public static Set <OrdT, T> except <OrdT, T>(Set <OrdT, T> setA, Set <OrdT, T> setB) where OrdT : struct, Ord <T> => setA.Except(setB);
public static Set <T> difference <T>(Set <T> setA, Set <T> setB) => setA.Except(setB);
public static Set <T> subtract <T>(Set <T> setA, Set <T> setB) => setA.Except(setB);