docs: @Override
      public DataSet get(int i) {
          if (i >= numExamples() || i < 0)
              throw new IllegalArgumentException("invalid example number: must be 0 to " + (numExamples()-1) + ", got " + i);
          if (i == 0 && numExamples() == 1)
              return this;
          return new DataSet(getHelper(features,i), getHelper(labels, i), getHelper(featuresMask,i), getHelper(labelsMask,i));
      }