Fixpoint hlist_get ls a (m : member a ls) : hlist ls -> F a :=
    match m in member _ ls return hlist ls -> F a with
      | MZ _ => hlist_hd
      | MN _ _ r => fun hl => hlist_get r (hlist_tl hl)
    end.Fixpoint hlist_nth_error {ls} (hs : hlist ls) (n : nat)
    : option (match nth_error ls n with
                | None => unit
                | Some x => F x
              end) :=
    match hs in hlist ls return option (match nth_error ls n with
                                          | None => unit
                                          | Some x => F x
                                        end)
      with
      | Hnil => None
      | Hcons l ls h hs =>
        match n as n return option (match nth_error (l :: ls) n with
                                      | None => unit
                                      | Some x => F x
                                    end)
          with
          | 0 => Some h
          | S n => hlist_nth_error hs n
        end
    end.Polymorphic Fixpoint hlist_nth ls (h : hlist ls) (n : nat) :
    match nth_error ls n return Type with
      | None => unit
      | Some t => F t
    end :=
    match h in hlist ls , n as n
      return match nth_error ls n with
               | None => unit
               | Some t => F t
             end
      with
      | Hnil , 0 => tt
      | Hnil , S _ => tt
      | Hcons _ _ x _ , 0 => x
      | Hcons _ _ _ h , S n => hlist_nth h n
    end.Fixpoint nth_error_hlist_nth ls (n : nat)
  : option (hlist ls -> match nth_error ls n with
                          | None => Empty_set
                          | Some x => F x
                        end) :=
    match ls as ls
          return option (hlist ls -> match nth_error ls n with
                                       | None => Empty_set
                                       | Some x => F x
                                     end)
    with
      | nil => None
      | l :: ls =>
        match n as n
              return option (hlist (l :: ls) -> match nth_error (l :: ls) n with
                                                  | None => Empty_set
                                                  | Some x => F x
                                                end)
        with
          | 0 => Some hlist_hd
          | S n =>
            match nth_error_hlist_nth ls n with
              | None => None
              | Some f => Some (fun h => f (hlist_tl h))
            end
        end
    end.Definition cast1 T l
  : forall (l' : list T) n v,
      nth_error l n = Some v -> Some v = nth_error (l ++ l') n.Definition cast2 T l
  : forall (l' : list T) n,
      nth_error l n = None ->
      nth_error l' (n - length l) = nth_error (l ++ l') n.

Theorem hlist_nth_hlist_app : forall l l' (h : hlist l) (h' : hlist l') n, hlist_nth (hlist_app h h') n = match nth_error l n as k return nth_error l n = k -> match nth_error (l ++ l') n return Type with | None => unit | Some t => F t end with | Some _ => fun pf => match cast1 _ _ _ pf in _ = z , eq_sym pf in _ = w return match w return Type with | None => unit | Some t => F t end -> match z return Type with | None => unit | Some t => F t end with | eq_refl , eq_refl => fun x => x end (hlist_nth h n) | None => fun pf => match cast2 _ _ _ pf in _ = z return match z with | Some t => F t | None => unit end with | eq_refl => hlist_nth h' (n - length l) end end eq_refl.