Inductive tree X : Type :=
| leaf
| node (data: X) (o_l o_r : tree X).
Arguments leaf {_}.
Arguments node {_} _ _ _.
Fixpoint is_p_b_tr {X} (o : tree X) (l : nat) : Prop :=
match o, l with
| leaf, O => True
| node _ o_l o_r, S l' =>
is_p_b_tr o_l l' /\ is_p_b_tr o_r l'
| _, _ => False
end.
Definition path := list bool.
Fixpoint coord_in_bound {X} (o : tree X) (p : path) (stop: nat) : Prop :=
match o with
| leaf => False
| node d_o o_l o_r =>
match stop with
| 0%nat => True
| S stop' =>
match p with
| [] => False
| true :: xs => coord_in_bound o_l xs stop'
| false :: xs => coord_in_bound o_r xs stop'
end
end
end.

Lemma pb_coord_in_bound {X} : forall (o : tree X) (p : path) (k lvl : nat),
is_p_b_tr o (S k) ->
(length p) = k ->
lvl <= k ->
coord_in_bound o p lvl.