∀ a0. a0 ∈ ∅ ++ a ++ b ↔ a0 ∈ ∅ ++ c ++ b ⊦ a = c, b = c
Used simplifier rules: add: ⊦ a ∈ s ++ b ↔ ¬(a ≠ b ∧ ¬ a ∈ s);%In-empty: ⊦ ¬ a ∈ ∅;