Used simplifier rules: undel-01: ⊦ (s ∪ s0) -- a = s ∪ (s0 -- a) ↔ ¬ a ∈ s;%una: ⊦ (s ∪ s0) ∪ s1 = s ∪ s0 ∪ s1;%unc: ⊦ s ∪ s0 = s0 ∪ s;