code: public double recall() {
      if(!isBinary())throw new UnsupportedOperationException("recall is only implemented for 2 class problems.");
      double tp = _arr[1][1];
      double fn = _arr[1][0];
      return tp / (tp + fn);
    }
docs: The percentage of positive labeled instances that were predicted as positive.
  @return Recall / TPR / Sensitivity