∀ b, a. # s0 < 2 ∧ b ∈ s0 ∧ a ∈ s0 → a = b, #(s0 ++ a0) < 2, b ∈ s0 ++ a0, a ∈ s0 ++ a0 ⊦ a = b
Used simplifier rules: gen: ⊦ ;%Size-empty: ⊦ # ∅ = 0;%In-empty: ⊦ ¬ a ∈ ∅;