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