(* In_tree *)
(* ------- *)

Inductive In_tree : A -> Tree -> Prop :=
  | in_leave : forall (a : A) (succs : Forest), In_tree a (node a succs)
  | in_succs :
      forall (succs : Forest) (t : Tree),
      In_Forest t succs ->
      forall a : A, In_tree a t -> forall a' : A, In_tree a (node a' succs).(* In_tree *)
(* ------- *)

Inductive In_tree : A -> Tree -> Prop :=
  | in_leave : forall (a : A) (succs : Forest), In_tree a (node a succs)
  | in_succs :
      forall (succs : Forest) (t : Tree),
      In_Forest t succs ->
      forall a : A, In_tree a t -> forall a' : A, In_tree a (node a' succs).(* In_tree *)
(* ------- *)

Inductive In_tree : A -> Tree -> Prop :=
  | in_leave : forall (a : A) (succs : Forest), In_tree a (node a succs)
  | in_succs :
      forall (succs : Forest) (t : Tree),
      In_Forest t succs ->
      forall a : A, In_tree a t -> forall a' : A, In_tree a (node a' succs).

Lemma in_successor_in : forall (a : A) (t : Tree), In_tree a t -> forall t' : Tree, Successor t t' -> In_tree a t'.