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