#(s0 ++ a0) < 2, a ∈ s0, ∀ a, b. # s0 < 2 ∧ a ∈ s0 ∧ b ∈ s0 → a = b, b = a0 ∨ b ∈ s0 ⊦ b = a
Used formulas: b = a