∀ b. b ∈ s1 ↔ b = a ∨ b ∈ s0 ⊦ a0 ∈ s1 ↔ a0 = a ∨ a0 ∈ s0
Used simplifier rules: In-insert: ⊦ a ∈ s ++ b ↔ a = b ∨ a ∈ s;