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