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