Inductive fold_rel (A: Type) (f: A -> A -> A -> Prop):  A -> list A -> A -> Prop :=
| fold_rel_nil: forall w, fold_rel f w nil w
| fold_rel_cons: forall a l w w1 w2,
            f w a w1 ->
            fold_rel f w1 l w2 ->
            fold_rel f w (a::l) w2.Inductive fold_rel (A: Type) (f: A -> A -> A -> Prop):  A -> list A -> A -> Prop :=
| fold_rel_nil: forall w, fold_rel f w nil w
| fold_rel_cons: forall a l w w1 w2,
            f w a w1 ->
            fold_rel f w1 l w2 ->
            fold_rel f w (a::l) w2.Inductive fold_rel (A: Type) (f: A -> A -> A -> Prop):  A -> list A -> A -> Prop :=
| fold_rel_nil: forall w, fold_rel f w nil w
| fold_rel_cons: forall a l w w1 w2,
            f w a w1 ->
            fold_rel f w1 l w2 ->
            fold_rel f w (a::l) w2.Inductive list_forall2 {A B} (P: A -> B -> Prop): list A -> list B -> Prop :=
  | list_forall2_nil:
      list_forall2 P nil nil
  | list_forall2_cons:
      forall a1 al b1 bl,
      P a1 b1 ->
      list_forall2 P al bl ->
      list_forall2 P (a1 :: al) (b1 :: bl).Inductive list_forall2 {A B} (P: A -> B -> Prop): list A -> list B -> Prop :=
  | list_forall2_nil:
      list_forall2 P nil nil
  | list_forall2_cons:
      forall a1 al b1 bl,
      P a1 b1 ->
      list_forall2 P al bl ->
      list_forall2 P (a1 :: al) (b1 :: bl).Inductive list_forall2 {A B} (P: A -> B -> Prop): list A -> list B -> Prop :=
  | list_forall2_nil:
      list_forall2 P nil nil
  | list_forall2_cons:
      forall a1 al b1 bl,
      P a1 b1 ->
      list_forall2 P al bl ->
      list_forall2 P (a1 :: al) (b1 :: bl).Fixpoint replace (A: Type) (l: list A) (n:nat) (new: A) {struct n} : option (list A) :=
  match (l, n) with
    | (nil, n) => None
    | (x :: xs, O) => Some (new :: xs)
    | (x :: xs, S n) =>
      match (replace xs n new) with
        |  None => None
        |  Some xs => Some (x :: xs)
      end
  end.Definition list_join {A} {JA: Join A}: A -> list A -> A -> Prop :=
   fold_rel join.

Lemma list_join_app {A} {JA: Join A}: forall {a: A} {bl c dl e}, list_join a bl c -> list_join c dl e -> list_join a (bl++dl) e.