a0 ∈ s, ∀ a0. a0 ∈ s → a0 ∈ s0 ⊦ a ∈ s0, a ∈ s, a0 ∈ s0, s0 ++ a ⊆ s0
Used simplifier rules: ne-01: ⊦ a ∈ s → s ≠ ∅;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);