a ∈ s, # s ≠ 0 → a ∈ s ⊦ # s = 2 ↔ (∃ a, b. a ≠ b ∧ s = ∅ ++ a ++ b)
Used simplifier rules: size-zero-02: ⊦ # s = 0 → s = ∅;%Size-empty: ⊦ # ∅ = 0;%ne: ⊦ ∅ ≠ s ++ a; Used formulas: a ∈ s