c ∈ s ⊦ a0 = a, c = b, a ∈ s, a0 ∈ s, s ⊆ s ++ a ++ a0 ++ b ++ c
Used simplifier rules: In-insert: ⊦ a ∈ s ++ b ↔ a = b ∨ a ∈ s;