Theorem sb2ae 2530

Description: In the case of two successive substitutions for two always equal variables, the second substitution has no effect. Usage of this theorem is discouraged because it depends on ax-13 2406. (Contributed by BJ and WL, 9-Aug-2023.) (New usage is discouraged.)

Assertion

Ref	Expression
sb2ae	⊢ (∀𝑥 𝑥 = 𝑦 → ([𝑢 / 𝑥][𝑣 / 𝑦]𝜑 ↔ [𝑣 / 𝑦]𝜑))

Distinct variable group:   𝑦,𝑣

Allowed substitution hints:   𝜑(𝑥, 𝑦, 𝑣, 𝑢)

Proof of Theorem sb2ae

Step	Hyp	Ref	Expression
1		drsb1 2529	. 2 ⊢ (∀𝑥 𝑥 = 𝑦 → ([𝑢 / 𝑥][𝑣 / 𝑦]𝜑 ↔ [𝑢 / 𝑦][𝑣 / 𝑦]𝜑))
2		nfs1v 2194	. . 3 ⊢ Ⅎ𝑦[𝑣 / 𝑦]𝜑
3	2	sbf 2308	. 2 ⊢ ([𝑢 / 𝑦][𝑣 / 𝑦]𝜑 ↔ [𝑣 / 𝑦]𝜑)
4	1, 3	bitrdi 290	1 ⊢ (∀𝑥 𝑥 = 𝑦 → ([𝑢 / 𝑥][𝑣 / 𝑦]𝜑 ↔ [𝑣 / 𝑦]𝜑))

This proof depends on syntax axioms:   → wi 4   ↔ wb 209  ∀wal 1568  [wsb 2099

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-10 2179  ax-12 2216  ax-13 2406

This proof depends on definitions:  df-bi 210  df-an 402  df-or 862  df-ex 1813  df-nf 1817  df-sb 2100

This theorem is used by: (None)