public static S foldBack <T, S>(Set <T> set, S state, Func <S, T, S> folder) => set.FoldBack(state, folder);
public static S foldBack <OrdT, T, S>(Set <OrdT, T> set, S state, Func <S, T, S> folder) where OrdT : struct, Ord <T> => set.FoldBack(state, folder);