1 < # s0 → (∃ a, b. a ≠ b ∧ a ∈ s0 ∧ b ∈ s0), 1 < #(s0 ++ a) ⊦ ∃ a0, b. a0 ≠ b ∧ a0 ∈ s0 ++ a ∧ b ∈ s0 ++ a
Used simplifier rules: gen: ⊦ ;%Size-empty: ⊦ # ∅ = 0;