c ∈ s ⊦ c = b, a0 = a, a ∈ s, s ⊆ s ++ a ++ a0 ++ b ++ c, a0 ∈ s
Used simplifier rules: add: ⊦ ¬ a ∈ s ++ b ↔ a ≠ b ∧ ¬ a ∈ s;