code: @Override
    double getEstimate() {
      final int couponCount = getCouponCount();
      final double est = CubicInterpolation.usingXAndYTables(CouponMapping.xArr,
          CouponMapping.yArr, couponCount);
      return max(est, couponCount);
    }
docs: This is the estimator for the Coupon List mode and Coupon Hash Set mode.
  
  <p>Note: This is an approximation to the true mapping from numCoupons to N,
  which has a range of validity roughly from 0 to 6 million coupons.</p>
  
  <p>The k of the implied coupon sketch, which must not be confused with the k of the HLL
  sketch.  In this application k is always 2^26, which is the number of address bits of the
  32-bit coupon.</p>
  @return the unique count estimate.