intro; unfold xi, vi, thetat in |- *.
assert (H := cond_x intr t).
rewrite <- H.
apply pr_nu.
Qed.

(* Specifications: *)
Lemma Dy :
forall t : R,
derive_pt (yi intr) t (yi_derivable intr t) =
(vi intr * sin (thetat intr t))%R.