⊦ a ∈ s ↔ (∃ s0. s = s0 ++ a ∧ ¬ a ∈ s0)
Used simplifier rules: Sep-ins: ⊦ ¬ a ∈ s → aP ⊗_ (s ++ a) = aP(a) * aP ⊗_ s;% ⊦ ¬ a ∈ s → (s ++ a) -- a = s; (s) Used terms: s, a