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