Theorem wl-equsal 35436

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 35435 first, and then deriving more specialized versions wl-equsal 35436 and wl-equsal1t 35437 then is more efficient than the other way round, which is possible, too. See also equsal 2416. (Revised by Wolf Lammen, 27-Jul-2019.) (Proof modification is discouraged.)

Hypotheses

Ref	Expression
wl-equsal.1	⊢ Ⅎ𝑥𝜓
wl-equsal.2	⊢ (𝑥 = 𝑦 → (𝜑 ↔ 𝜓))

Assertion

Ref	Expression
wl-equsal	⊢ (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ 𝜓)

Proof of Theorem wl-equsal

Step	Hyp	Ref	Expression
1		nftru 1812	. . 3 ⊢ Ⅎ𝑥⊤
2		wl-equsal.1	. . . 4 ⊢ Ⅎ𝑥𝜓
3	2	a1i 11	. . 3 ⊢ (⊤ → Ⅎ𝑥𝜓)
4		wl-equsal.2	. . . 4 ⊢ (𝑥 = 𝑦 → (𝜑 ↔ 𝜓))
5	4	a1i 11	. . 3 ⊢ (⊤ → (𝑥 = 𝑦 → (𝜑 ↔ 𝜓)))
6	1, 3, 5	wl-equsald 35435	. 2 ⊢ (⊤ → (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ 𝜓))
7	6	mptru 1550	1 ⊢ (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ 𝜓)

Syntax hints:   → wi 4   ↔ wb 209  ∀wal 1541  ⊤wtru 1544  Ⅎwnf 1791

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1803  ax-4 1817  ax-5 1918  ax-6 1976  ax-7 2016  ax-12 2175  ax-13 2371

This theorem depends on definitions:  df-bi 210  df-an 400  df-tru 1546  df-ex 1788  df-nf 1792

This theorem is referenced by: (None)