Definition ohead s := if s is x :: _ then Some x else None.Definition head s := if s is x :: _ then x else x0.Definition behead s := if s is _ :: s' then s' else [::].(* Factories *)

Definition ncons n x := iter n (cons x).Definition nseq n x := ncons n x [::].

Lemma size_ncons n x s : size (ncons n x s) = n + size s.