Used simplifier rules: One: ⊦ {a} = ∅ ++ a;%unadd: ⊦ (s ++ a) ∪ s0 = (s ∪ s0) ++ a;%une: ⊦ s ∪ ∅ = s;%una: ⊦ (s ∪ s0) ∪ s1 = s ∪ s0 ∪ s1;%unc: ⊦ s ∪ s0 = s0 ∪ s;% ⊦ a ∈ s ++ b ↔ a = b ∨ a ∈ s; (ws)