∀ a0. a0 ∈ s ++ a ↔ a0 ∈ s0 ++ b ⊦ a = b, a ∈ s0
Used simplifier rules: Extension: ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2);