a ∈ s ⊦ ∃ s0. s = s0 ++ a ∧ ¬ a ∈ s0
Used simplifier rules: ne-01: ⊦ a ∈ s → s ≠ ∅;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);