|Description: Axiom of Quantifier
Substitution. One of the equality and substitution
axioms of predicate calculus with equality. Appears as Lemma L12 in
[Megill] p. 445 (p. 12 of the preprint).
The original version of this axiom was ax-10o 1836 ("o" for "old") and was
replaced with this shorter ax-10 1678 in May 2008. The old axiom is proved
from this one as theorem ax10o 1835. Conversely, this axiom is proved from
ax-10o 1836 as theorem ax10from10o 1837.
This axiom was proved redundant in July 2015. See theorem ax10 1677.
(Contributed by NM, 16-May-2008.)