(** Having [NoDup] hypotheses bring more precise facts about [incl]. *)

(* Specifications: *)
Lemma NoDup_incl_length l l' :
NoDup l -> incl l l' -> length l <= length l'.
Lemma NoDup_length_incl l l' :
NoDup l -> length l' <= length l -> incl l l' -> incl l' l.
Lemma NoDup_incl_NoDup (l l' : list A) : NoDup l ->
length l' <= length l -> incl l l' -> NoDup l'.