Well, you did
notice that I suggested that using DC connected through
a DPDT reversing switch
flipped once every fortnight or so would
satisfy the requirements, since the frequency of the AC wasn't
specified.
Yes, but as there;'s only 1 Vff pin (presumably referenced to ground),
you need a floating DC supply to do that. That's alomost as hard to get
as a true AC supply in a lot of cases.
-tony