a = b ⊦ (∀ a0. a0 = b ∨ ¬ a0 ∈ s ++ a) ↔ (∀ a0. ¬ a0 ∈ s) ∧ a = b, a ∈ s
⊦ (∀ a0. a0 = b ∨ ¬ a0 ∈ s ++ a) ↔ (∀ a0. ¬ a0 ∈ s) ∧ a = b, a = b, a ∈ s
Used formulas: a = b