# s = 1, a ∈ s ⊦ s = ∅ ++ a, a ∈ ∅
Used simplifier rules: eq: ⊦ a ∈ s → s ++ a = s;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);%In-empty: ⊦ ¬ a ∈ ∅; Used formulas: a ∈ ∅