Description: If dtru is false, then there is only one element in the universe, so
everything satisfies F/ . (Contributed by Mario Carneiro, 7-Oct-2016) Remove dependency on ax-11 . (Revised by Wolf Lammen, 9-Sep-2018)(Proof shortened by BJ, 14-Jun-2019) Remove dependency
on ax-10 . (Revised by Wolf lammen, 12-Oct-2021)