x = a + y, dups(x) ⊦ x = [], (a ∈ y ∨ dups(y)) ∧ # x ≠ 1
Used simplifier rules: x ≠ [] ⊦ a = x.head ∧ y = x.tail ↔ x = a + y; (elim) Used terms: x, a, y