public static HashSet <T> symmetricExcept <T>(HashSet <T> setA, IEnumerable <T> setB) => setA.SymmetricExcept(setB);
public static HashSet <EqT, T> symmetricExcept <EqT, T>(HashSet <EqT, T> setA, HashSet <EqT, T> setB) where EqT : struct, Eq <T> => setA.SymmetricExcept(setB);