a ∈ y, a ∈ (a0 ' + x0) \ y, ∀ a1, y0. ¬ a1 ∈ x0 \ y0 ∨ ¬ a1 ∈ y0 ⊦
Used simplifier rules: c: ⊦ a + x = a ' + x; from specification list