s0 ++ a = ∅ ++ a0 ++ b ⊦ 1 = # s0, a0 = b, s0 = ∅ ++ b, a ∈ s0, ∃ a. s0 = ∅ ++ a
Used simplifier rules: In-empty: ⊦ ¬ a ∈ ∅;