Theorem ax9ALT 2756

Description: Proof of ax-9 2155 from Tarski's FOL and dfcleq 2754. For a version not using ax-8 2147 either, see eleq2w2 2757. This shows that dfcleq 2754 is too powerful to be used as a definition instead of df-cleq 2753. Note that ax-ext 2733 is also a direct consequence of dfcleq 2754 (as an instance of its forward implication). (Contributed by BJ, 24-Jun-2019.) (Proof modification is discouraged.) (New usage is discouraged.)

Assertion

Ref	Expression
ax9ALT	⊢ (𝑥 = 𝑦 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))

Proof of Theorem ax9ALT

Dummy variable 𝑡 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		dfcleq 2754	. . . 4 ⊢ (𝑥 = 𝑦 ↔ ∀𝑡(𝑡 ∈ 𝑥 ↔ 𝑡 ∈ 𝑦))
2	1	biimpi 219	. . 3 ⊢ (𝑥 = 𝑦 → ∀𝑡(𝑡 ∈ 𝑥 ↔ 𝑡 ∈ 𝑦))
3		biimp 218	. . 3 ⊢ ((𝑡 ∈ 𝑥 ↔ 𝑡 ∈ 𝑦) → (𝑡 ∈ 𝑥 → 𝑡 ∈ 𝑦))
4	2, 3	sylg 1856	. 2 ⊢ (𝑥 = 𝑦 → ∀𝑡(𝑡 ∈ 𝑥 → 𝑡 ∈ 𝑦))
5		ax8 2151	. . . . 5 ⊢ (𝑧 = 𝑡 → (𝑧 ∈ 𝑥 → 𝑡 ∈ 𝑥))
6	5	equcoms 2053	. . . 4 ⊢ (𝑡 = 𝑧 → (𝑧 ∈ 𝑥 → 𝑡 ∈ 𝑥))
7		ax8 2151	. . . 4 ⊢ (𝑡 = 𝑧 → (𝑡 ∈ 𝑦 → 𝑧 ∈ 𝑦))
8	6, 7	imim12d 82	. . 3 ⊢ (𝑡 = 𝑧 → ((𝑡 ∈ 𝑥 → 𝑡 ∈ 𝑦) → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦)))
9	8	spimvw 2019	. 2 ⊢ (∀𝑡(𝑡 ∈ 𝑥 → 𝑡 ∈ 𝑦) → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))
10	4, 9	syl 18	1 ⊢ (𝑥 = 𝑦 → (𝑧 ∈ 𝑥 → 𝑧 ∈ 𝑦))

This proof depends on syntax axioms:   → wi 4   ↔ wb 209  ∀wal 1568

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1828  ax-4 1842  ax-5 1943  ax-6 2000  ax-7 2041  ax-8 2147  ax-9 2155  ax-ext 2733

This proof depends on definitions:  df-bi 210  df-an 402  df-ex 1813  df-cleq 2753

This theorem is used by: (None)