set (xi_p := xi intr); set (yi_p := yi intr); reg.
apply (yi_derivable intr).
apply (xi_derivable intr).
Qed.
Hint Resolve Rsqr_evader_distance_derivable: diff.
Definition Rsqr_intruder_distance (t : R) : R :=
(Rsqr (xi intr t - xi intr 0) + Rsqr (yi intr t - yi intr 0))%R.

Lemma Rsqr_intruder_distance_derivable : derivable Rsqr_intruder_distance.
unfold Rsqr_intruder_distance in |- *; set (xi_p := xi intr);
set (yi_p := yi intr); reg.