⊦ ordered<(a ' + y) ↔ a < y.head ∧ ordered<(y), y = []
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ a ' + x ≠ []; (s)% ⊦ a + x = a ' + x; (s)