(* Compute the number of nodes of a tree *) 
Fixpoint number_of_nodes (b : btree) : nat :=
  match b with
  | leaf _ => 0
  | node b1 b2 => S (number_of_nodes b1 + number_of_nodes b2)
  end.

Theorem number_of_nodes_inb_le : forall t1 t2, inb t1 t2 -> number_of_nodes t1 <= number_of_nodes t2.