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Theorem bj-dfssb2 33146
 Description: An alternate definition of df-ssb 33127. Note that the use of a dummy variable in the definition df-ssb 33127 allows to use bj-sb56 33145 instead of equs45f 2466 and hence to avoid dependency on ax-13 2377 and to use ax-12 2213 only through bj-ax12 33141. Compare dfsb7 2575. (Contributed by BJ, 25-Dec-2020.)
Assertion
Ref Expression
bj-dfssb2 ([𝑡/𝑥]b𝜑 ↔ ∃𝑦(𝑦 = 𝑡 ∧ ∃𝑥(𝑥 = 𝑦𝜑)))
Distinct variable groups:   𝑦,𝑡   𝑥,𝑦   𝜑,𝑦
Allowed substitution hints:   𝜑(𝑥,𝑡)

Proof of Theorem bj-dfssb2
StepHypRef Expression
1 df-ssb 33127 . 2 ([𝑡/𝑥]b𝜑 ↔ ∀𝑦(𝑦 = 𝑡 → ∀𝑥(𝑥 = 𝑦𝜑)))
2 bj-sb56 33145 . 2 (∃𝑦(𝑦 = 𝑡 ∧ ∀𝑥(𝑥 = 𝑦𝜑)) ↔ ∀𝑦(𝑦 = 𝑡 → ∀𝑥(𝑥 = 𝑦𝜑)))
3 bj-sb56 33145 . . . . 5 (∃𝑥(𝑥 = 𝑦𝜑) ↔ ∀𝑥(𝑥 = 𝑦𝜑))
43bicomi 216 . . . 4 (∀𝑥(𝑥 = 𝑦𝜑) ↔ ∃𝑥(𝑥 = 𝑦𝜑))
54anbi2i 617 . . 3 ((𝑦 = 𝑡 ∧ ∀𝑥(𝑥 = 𝑦𝜑)) ↔ (𝑦 = 𝑡 ∧ ∃𝑥(𝑥 = 𝑦𝜑)))
65exbii 1944 . 2 (∃𝑦(𝑦 = 𝑡 ∧ ∀𝑥(𝑥 = 𝑦𝜑)) ↔ ∃𝑦(𝑦 = 𝑡 ∧ ∃𝑥(𝑥 = 𝑦𝜑)))
71, 2, 63bitr2i 291 1 ([𝑡/𝑥]b𝜑 ↔ ∃𝑦(𝑦 = 𝑡 ∧ ∃𝑥(𝑥 = 𝑦𝜑)))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 198   ∧ wa 385  ∀wal 1651  ∃wex 1875  [wssb 33126 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1891  ax-4 1905  ax-5 2006  ax-6 2072  ax-7 2107  ax-10 2185  ax-12 2213 This theorem depends on definitions:  df-bi 199  df-an 386  df-ex 1876  df-ssb 33127 This theorem is referenced by:  bj-ssbn  33147
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