Theorem wl-equsal 35771

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 35770 first, and then deriving more specialized versions wl-equsal 35771 and wl-equsal1t 35772 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 1805	. . 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 35770	. 2 ⊢ (⊤ → (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ 𝜓))
7	6	mptru 1547	1 ⊢ (∀𝑥(𝑥 = 𝑦 → 𝜑) ↔ 𝜓)

Syntax hints:   → wi 4   ↔ wb 205  ∀wal 1538  ⊤wtru 1541  Ⅎwnf 1784

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1796  ax-4 1810  ax-5 1912  ax-6 1970  ax-7 2010  ax-12 2170  ax-13 2371

This theorem depends on definitions:  df-bi 206  df-an 397  df-tru 1543  df-ex 1781  df-nf 1785

This theorem is referenced by: (None)