EXERCISE 4.6 Show that the following two statements are equivalent (for any fixed formula A ): (1.a) for every :NA (1.b) NA, i.e., for being the empty theory: NA. Give an example of ,A and B showing the difference between (nonequivalence of) the following two equivalences: (2.a) for every :NA iff NB. (2.b)NA iff NB