public static HashSet <T> union <T>(HashSet <T> setA, HashSet <T> setB) => setA.Union(setB);
public static HashSet <EqT, T> union <EqT, T>(HashSet <EqT, T> setA, HashSet <EqT, T> setB) where EqT : struct, Eq <T> => setA.Union(setB);