n < #(a + x0), dups(lastn(n, a + x0)), ∀ n. n < # x0 ∧ dups(lastn(n, x0)) → dups(x0) ⊦ dups(a + x0)
Used simplifier rules: list freely generated by [], +;;% ⊦ # [] = 0; (s)% ⊦ ¬ n < 0; (s)