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