code: T getQuantile(final double fRank) {
      checkFractionalRankBounds(fRank);
      if (auxN_ <= 0) { return null; }
      final long pos = QuantilesHelper.posOfPhi(fRank, auxN_);
      return approximatelyAnswerPositionalQuery(pos);
    }
docs: Get the estimated quantile given a fractional rank.
  @param fRank the fractional rank where: 0 &le; fRank &le; 1.0.
  @return the estimated quantile