code: @Override
    public double distance(NumberVector v1, NumberVector v2) {
      return 1 - Math.abs(PearsonCorrelation.coefficient(v1, v2));
    }
docs: Computes the absolute Pearson correlation distance for two given feature
  vectors.
  
  The absolute Pearson correlation distance is computed from the Pearson
  correlation coefficient <code>r</code> as: <code>1-abs(r)</code>. Hence,
  possible values of this distance are between 0 and 1.
  
  @param v1 first feature vector
  @param v2 second feature vector
  @return the absolute Pearson correlation distance for two given feature
  vectors v1 and v2