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