Description: Modus ponendo tollens 2, one of the "indemonstrables" in Stoic logic.
Note that this uses exclusive-or \/_ . See rule 2 on
Lopez-Astorga p. 12 , rule 4 on Sanford p. 39 and rule A4 in
Hitchcock p. 5 . (Contributed by David A. Wheeler, 3-Jul-2016)(Proof shortened by Wolf Lammen, 12-Nov-2017)(Proof shortened by BJ, 19-Apr-2019)