public static bool exists <T>(HashSet <T> set, Func <T, bool> pred) => set.Exists(pred);
public static bool exists <EqT, T>(HashSet <EqT, T> set, Func <T, bool> pred) where EqT : struct, Eq <T> => set.Exists(pred);
public static bool exists <T>(HashSet <T> set, Func <T, bool> pred) => set.Exists(pred);
public static bool exists <EqT, T>(HashSet <EqT, T> set, Func <T, bool> pred) where EqT : struct, Eq <T> => set.Exists(pred);