Description: Version of drnf2 with a disjoint variable condition, which does not
require ax-10 , ax-11 , ax-12 , ax-13 . Instance of nfbidv .
Note that the version of axc15 with a disjoint variable condition is
actually ax12v2 (up to adding a superfluous antecedent). (Contributed by BJ, 17-Jun-2019)(Proof modification is discouraged.)