Definition fmono (f: nat → nat):= ∀ x y, x ≤ y → f x ≤ f y.

Lemma app_insert_r (A : Type) (l1 l1' l l2 l2' : list A): l1 ++ l2 = l1' ++ l2' -> length l2 = length l2' -> l1 ++ l ++ l2 = l1' ++ l ++ l2'.