apply (yi_derivable intr).
apply (xi_derivable intr).
Qed.
Hint Resolve xp_derivable: diff.
Definition Rsqr_evader_distance (t : R) : R :=
(Rsqr (xi intr t - xe) + Rsqr (yi intr t - ye))%R.

Lemma Rsqr_evader_distance_derivable : derivable Rsqr_evader_distance.
unfold Rsqr_evader_distance in |- *.