c ∈ s ⊦ a ∈ s, c = a, c = a0, c = b, s ⊆ s ++ a ++ a0 ++ b ++ c
Used simplifier rules: eq: ⊦ a ∈ s → s ++ a = s;%subset-insert: ⊦ s ⊆ s ++ a ++ b ++ c; Used formulas: a ∈ s