intro; reg. apply (cond_D1 alphas).
apply Rs_derivable.
Qed.

(**********)
Lemma Dxi :
forall t : R,
derive_pt (xi intr) t (xi_derivable intr t) =
(derive_pt Rs t (Rs_derivable t) * cos (alphas t) -
Rs t * sin (alphas t) * derive_pt alphas t (cond_D1 alphas t))%R.