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Theorem wl-equsal 32505
 Description: A useful equivalence related to substitution. (Contributed by NM, 2-Jun-1993.) (Proof shortened by Andrew Salmon, 12-Aug-2011.) (Revised by Mario Carneiro, 3-Oct-2016.) It seems proving wl-equsald 32504 first, and then deriving more specialized versions wl-equsal 32505 and wl-equsal1t 32506 then is more efficient than the other way round, which is possible, too. See also equsal 2279. (Revised by Wolf Lammen, 27-Jul-2019.) (Proof modification is discouraged.)
Hypotheses
Ref Expression
wl-equsal.1 𝑥𝜓
wl-equsal.2 (𝑥 = 𝑦 → (𝜑𝜓))
Assertion
Ref Expression
wl-equsal (∀𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)

Proof of Theorem wl-equsal
StepHypRef Expression
1 nftru 1721 . . 3 𝑥
2 wl-equsal.1 . . . 4 𝑥𝜓
32a1i 11 . . 3 (⊤ → Ⅎ𝑥𝜓)
4 wl-equsal.2 . . . 4 (𝑥 = 𝑦 → (𝜑𝜓))
54a1i 11 . . 3 (⊤ → (𝑥 = 𝑦 → (𝜑𝜓)))
61, 3, 5wl-equsald 32504 . 2 (⊤ → (∀𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓))
76trud 1484 1 (∀𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 195  ∀wal 1473  ⊤wtru 1476  Ⅎwnf 1699 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1713  ax-4 1728  ax-5 1827  ax-6 1875  ax-7 1922  ax-12 2034  ax-13 2234 This theorem depends on definitions:  df-bi 196  df-or 384  df-an 385  df-tru 1478  df-ex 1696  df-nf 1701 This theorem is referenced by: (None)
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