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_pos : forall t : R, (0 <= Rsqr_intruder_distance t)%R.