⊦ rmdup((a ' + x) + y) = a ' + rmdup(x + y), a ∈ y, a ∈ x
Used simplifier rules: union: ⊦ x ∪ y = rmdup(x + y);%assoc: ⊦ (x ∪ y) ∪ z = x ∪ y ∪ z;% ⊦ a + x = a ' + x; (s)