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Definition Rsqr_Rmin (intruder evader : State) : R :=
  match Req_EM_var (Ax2_R intruder) 0 with
  | left _ => 0%R
  | right _ =>
      ((4 * Ax2_R intruder * C_R intruder evader -
        Rsqr (Bx_R intruder evader)) / (4 * Ax2_R intruder))%R
  end.

Lemma tau_equal_when_zero : forall (intruder evader : State) (t : R), div_tau intruder evader = 0%R -> tau intruder evader t = 0%R.