Used simplifier rules: in: ⊦ a ∈ list2set(x) ↔ a ∈ x;% ⊦ s ∪ s0 = s0 ∪ s;% ⊦ (s ∪ s0) ∪ s1 = s ∪ s0 ∪ s1;% ⊦ a ∈ s1 ∪ s2 ↔ a ∈ s1 ∨ a ∈ s2;