Used simplifier rules: union-sizele-01: ⊦ #((s ∪ s0) ++ a) ≤ # s ↔ s0 ⊆ s ∧ a ∈ s;%una: ⊦ (s ∪ s0) ∪ s1 = s ∪ s0 ∪ s1;%unc: ⊦ s ∪ s0 = s0 ∪ s;