b ∈ a + y0, b ∈ (a + y0) -1l b, ∀ b. b ∈ y0 -1l b ∧ b ∈ y0 → dups(y0) ⊦ dups(a + y0)
Used simplifier rules: del1-e: ⊦ [] -1l a = [];%list freely generated by [], +;;% ⊦ ¬ a ∈ []; (s)