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.