Fixpoint nodup (l : list A) : list A :=
    match l with
      | [] => []
      | x::xs => if in_dec decA x xs then nodup xs else x::(nodup xs)
    end.

Lemma seq_app : forall len1 len2 start, seq start (len1 + len2) = seq start len1 ++ seq (start + len1) len2.