⊦ a0 ∈ s ∩ (s0 ++ a) ↔ a0 ∈ s ∩ s0, a ∈ s
Used simplifier rules: Extension: ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2); from specification set-basic