Used simplifier rules: In-insert: ⊦ a ∈ s ++ b ↔ a = b ∨ a ∈ s;%eq-01: ⊦ ¬ a ∈ s ∧ ¬ a ∈ s0 → (s ++ a = s0 ++ a ↔ s = s0); Used terms: a