c ∈ s ⊦ a ∈ s, c = a, c = b, s ⊆ s ++ a ++ b ++ c
Used simplifier rules: eq: ⊦ a ∈ s → s ++ a = s;%ins-two: ⊦ a ∈ s → s ++ b ++ a = s ++ b;%sub-add-01: ⊦ s ⊆ s ++ a; Used formulas: a ∈ s