⊦ a ∈ s ↔ (∃ s0. s = s0 ++ a ∧ ¬ a ∈ s0)
∃ s1. s0 = s1 ++ a ∧ ¬ a ∈ s1, ∀ s. # s ≠ 0 → (∃ a0. a0 ∈ s) ⊦ # s0 = 0, # s0 = 1, ∃ a0. a0 ∈ s0 ∧ a0 ≠ a
Used terms: s0, a