∀ n. n < # x0 → pos(x0[n], x0) = n ∨ x0 = [] ∨ dups(x0), n < #(a + x0) ⊦ pos((a + x0)[n], a + x0) = n, a + x0 = [], dups(a + x0)
Used simplifier rules: list freely generated by [], +;;