Description: Version of 19.21 with a disjoint variable condition, requiring fewer
axioms.

Notational convention: We sometimes suffix with "v" the label of a
theorem using a distinct variable ("dv") condition instead of a
non-freeness hypothesis such as F/ x ph . Conversely, we sometimes
suffix with "f" the label of a theorem introducing such a non-freeness
hypothesis ("f" stands for "not free in", see df-nf ) instead of a
disjoint variable condition. For instance, 19.21v versus 19.21 and
vtoclf versus vtocl . Note that "not free in" is less restrictive
than "does not occur in". Note that the version with a disjoint
variable condition is easily proved from the version with the
corresponding non-freeness hypothesis, by using nfv . However, the dv
version can often be proved from fewer axioms. (Contributed by NM, 21-Jun-1993) Reduce dependencies on axioms. (Revised by Wolf Lammen, 2-Jan-2020)(Proof shortened by Wolf Lammen, 12-Jul-2020)