Description: Tarski's system uses the weaker ax6v instead of the bundled ax-6 , so
here we show that the degenerate case of ax-6 can be derived. Even
though ax-6 is in the list of axioms used, recall that in set.mm, the
only statement referencing ax-6 is ax6v . We later rederive from
ax6v the bundled form as ax6 with the help of the auxiliary axiom
schemes. (Contributed by NM, 23-Apr-2017)