a1 ∈ s, ∀ a0. ¬ a0 ∈ s ⊦ a0 ∈ s ↔ a0 = a, a1 = a
Used simplifier rules: ne-01: ⊦ a ∈ s → s ≠ ∅;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);