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Axiom ax-11o 1762
 Description: Axiom ax-11o 1762 ("o" for "old") was the original version of ax-11 1452, before it was discovered (in Jan. 2007) that the shorter ax-11 1452 could replace it. It appears as Axiom scheme C15' in [Megill] p. 448 (p. 16 of the preprint). It is based on Lemma 16 of [Tarski] p. 70 and Axiom C8 of [Monk2] p. 105, from which it can be proved by cases. To understand this theorem more easily, think of "¬ ∀𝑥𝑥 = 𝑦 →..." as informally meaning "if 𝑥 and 𝑦 are distinct variables, then..." The antecedent becomes false if the same variable is substituted for 𝑥 and 𝑦, ensuring the theorem is sound whenever this is the case. In some later theorems, we call an antecedent of the form ¬ ∀𝑥𝑥 = 𝑦 a "distinctor." This axiom is redundant, as shown by theorem ax11o 1761. This axiom is obsolete and should no longer be used. It is proved above as theorem ax11o 1761. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)
Assertion
Ref Expression
ax-11o (¬ ∀𝑥 𝑥 = 𝑦 → (𝑥 = 𝑦 → (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑))))

Detailed syntax breakdown of Axiom ax-11o
StepHypRef Expression
1 vx . . . . 5 setvar 𝑥
2 vy . . . . 5 setvar 𝑦
31, 2weq 1447 . . . 4 wff 𝑥 = 𝑦
43, 1wal 1297 . . 3 wff 𝑥 𝑥 = 𝑦
54wn 3 . 2 wff ¬ ∀𝑥 𝑥 = 𝑦
6 wph . . . 4 wff 𝜑
73, 6wi 4 . . . . 5 wff (𝑥 = 𝑦𝜑)
87, 1wal 1297 . . . 4 wff 𝑥(𝑥 = 𝑦𝜑)
96, 8wi 4 . . 3 wff (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑))
103, 9wi 4 . 2 wff (𝑥 = 𝑦 → (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑)))
115, 10wi 4 1 wff (¬ ∀𝑥 𝑥 = 𝑦 → (𝑥 = 𝑦 → (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑))))
 Colors of variables: wff set class This axiom is referenced by: (None)
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