(* QuickChick fst_swap_is_snd. *)

Inductive natlist : Type :=
  | nil  : natlist
  | cons : nat -> natlist -> natlist.(* QuickChick fst_swap_is_snd. *)

Inductive natlist : Type :=
  | nil  : natlist
  | cons : nat -> natlist -> natlist.(* QuickChick fst_swap_is_snd. *)

Inductive natlist : Type :=
  | nil  : natlist
  | cons : nat -> natlist -> natlist.Instance dec_natlist (p q : natlist) : Dec (p = q).Fixpoint repeat (n count : nat) : natlist :=
  match count with
  | O => nil
  | S count' => n :: (repeat n count')
  end.Fixpoint length (l:natlist) : nat :=
  match l with
  | nil => O
  | h :: t => S (length t)
  end.Fixpoint app (l1 l2 : natlist) : natlist :=
  match l1 with
  | nil    => l2
  | h :: t => h :: (app t l2)
  end.Definition hd (default:nat) (l:natlist) : nat :=
  match l with
  | nil => default
  | h :: t => h
  end.Definition tl (l:natlist) : natlist :=
  match l with
  | nil => nil
  | h :: t => t
  end.

Theorem app_assoc : forall l1 l2 l3 : natlist, (l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).