# s = # s1 + 1, a0 ∈ s, ∀ a0. a0 ∈ s → a0 ∈ s1 ⊦ a0 ∈ s1, a ∈ s1, a ∈ s
Used simplifier rules: ne-01: ⊦ a ∈ s → s ≠ ∅;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);