Remark list_norepet_append_commut:
  forall (A: Type) (a b: list A),
  list_norepet (a ++ b) -> list_norepet (b ++ a).

Lemma list_norepet_append_left: forall (A: Type) (l1 l2: list A), list_norepet (l1 ++ l2) -> list_norepet l1.