apply (yi_derivable intr).
apply (xi_derivable intr).
Qed.
Hint Resolve yp_derivable: diff.
Definition xp (t : R) : R :=
(cos (thetat intr 0) * (xi intr t - xe) +
sin (thetat intr 0) * (yi intr t - ye))%R.

Lemma xp_derivable : derivable xp.
unfold xp in |- *; set (thetat_p := thetat intr); set (xi_p := xi intr);
set (yi_p := yi intr); reg.