Used simplifier rules: undel-04: ⊦ (s -- a) ∪ (s0 -- a) = (s ∪ s0) -- a;%una: ⊦ (s ∪ s0) ∪ s1 = s ∪ s0 ∪ s1;%unc: ⊦ s ∪ s0 = s0 ∪ s;