public static HashSet <T> addOrUpdate <T>(HashSet <T> set, T value) => set.AddOrUpdate(value);
public static HashSet <EqT, T> addOrUpdate <EqT, T>(HashSet <EqT, T> set, T value) where EqT : struct, Eq <T> => set.AddOrUpdate(value);
public static HashSet <T> addOrUpdate <T>(HashSet <T> set, T value) => set.AddOrUpdate(value);
public static HashSet <EqT, T> addOrUpdate <EqT, T>(HashSet <EqT, T> set, T value) where EqT : struct, Eq <T> => set.AddOrUpdate(value);