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Axiom ax-12 1946
 Description: Axiom of Quantified Equality. One of the equality and substitution axioms of predicate calculus with equality. An equivalent way to express this axiom that may be easier to understand is (see ax12b 1697). Recall that in the intended interpretation, our variables are metavariables ranging over the variables of predicate calculus (the object language). In order for the first antecedent to hold, and must have different values and thus cannot be the same object-language variable. Similarly, and cannot be the same object-language variable. Therefore, will not occur in the wff when the first two antecedents hold, so analogous to ax-17 1623, the conclusion follows. The original version of this axiom was ax-12o 2192 and was replaced with this shorter ax-12 1946 in December 2015. The old axiom is proved from this one as theorem ax12o 1976. Conversely, this axiom is proved from ax-12o 2192 as theorem ax12 1985. The primary purpose of this axiom is to provide a way to introduce the quantifier on even when and are substituted with the same variable. In this case, the first antecedent becomes and the axiom still holds. Although this version is shorter, the original version ax12o 1976 may be more practical to work with because of the "distinctor" form of its antecedents. A typical application of ax12o 1976 is in dvelimh 2015 which converts a distinct variable pair to the distinctor antecendent . This axiom can be weakened if desired by adding distinct variable restrictions on pairs and . To show that, we add these restrictions to theorem ax12v 1947 and use only ax12v 1947 for further derivations. Thus, ax12v 1947 should be the only theorem referencing this axiom. Other theorems can reference either ax12v 1947 or ax12o 1976. This axiom scheme is logically redundant (see ax12w 1735) but is used as an auxiliary axiom to achieve metalogical completeness. (Contributed by NM, 21-Dec-2015.) (New usage is discouraged.)
Assertion
Ref Expression
ax-12

Detailed syntax breakdown of Axiom ax-12
StepHypRef Expression
1 vx . . . 4
2 vy . . . 4
31, 2weq 1650 . . 3
43wn 3 . 2
5 vz . . . 4
62, 5weq 1650 . . 3
76, 1wal 1546 . . 3
86, 7wi 4 . 2
94, 8wi 4 1
 Colors of variables: wff set class This axiom is referenced by:  ax12v  1947
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