a ∈ y, dups(x0 \ y), ∀ y0. ¬ dups(x0 \ y0) ⊦ a ∈ x0, dups(x0)
Used simplifier rules: diff-y: ⊦ a ∈ y → (a ' + x) \ y = x \ y;%a: ⊦ (x + y) + z = x + y + z; from specification list