(** Variant to prove the termination of Floyd's algorithm: - either [x m] has not entered the loop yet and [lambda-m] is decreasing - or bot [x m] and [x (2m)] are inside the loop and the distance between [x (2m)] and [x m] is decreasing Thus we use a lexicographic order. *)
Definition lex x y := fst x < fst y \/ (fst x = fst y /\ snd x < snd y).
(** Correctness proof *)

Theorem rec_call_is_wf :
forall m, x m <> x (2*m) -> lex (variant (S m)) (variant m).