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Theorem ax9v 1987
 Description: Weakened version of ax-9 1986, with a dv condition on 𝑥, 𝑦. This should be the only proof referencing ax-9 1986, and it should be referenced only by its two weakened versions ax9v1 1988 and ax9v2 1989, from which ax-9 1986 is then rederived as ax9 1990, which shows that either ax9v 1987 or the conjunction of ax9v1 1988 and ax9v2 1989 is sufficient. (Contributed by BJ, 7-Dec-2020.) Use ax9 1990 instead. (New usage is discouraged.)
Assertion
Ref Expression
ax9v (𝑥 = 𝑦 → (𝑧𝑥𝑧𝑦))
Distinct variable group:   𝑥,𝑦

Proof of Theorem ax9v
StepHypRef Expression
1 ax-9 1986 1 (𝑥 = 𝑦 → (𝑧𝑥𝑧𝑦))
 Colors of variables: wff setvar class Syntax hints:   → wi 4 This theorem was proved from axioms:  ax-9 1986 This theorem is referenced by:  ax9v1  1988  ax9v2  1989
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