(* Fibonacci numbers *)

(* definition *)

Fixpoint Fib (n : nat) : nat :=
  match n return nat with
  | O => (* O *)  1
      (* (S p ) *) 
  | S p =>
      match p return nat with
      | O => (* O *)  1
          (* S q *)
      | S q => Fib q + Fib p
      end
  end.

Lemma Unfold_FibSSn : forall n : nat, Fib (S (S n)) = Fib (S n) + Fib n.