# s < # s0 ⊦ ∃ a. a ∈ s0 ∧ ¬ a ∈ s
Used simplifier rules: ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);%sub-in: ⊦ a ∈ s ∧ s ⊆ s0 → a ∈ s0; Used terms: s0, s