∀ a, y. a ∈ x0 ∧ a ∈ y → dups(x0 + y), a ∈ a0 + x0, a ∈ y ⊦ dups((a0 + x0) + y)
Used simplifier rules: list freely generated by [], +;;%in-empty: ⊦ ¬ a ∈ []; from specification list