code: @Override
    public double distance(NumberVector v1, NumberVector v2) {
      return 1 - PearsonCorrelation.coefficient(v1, v2);
    }
docs: Computes the Pearson correlation distance for two given feature vectors.
  
  The Pearson correlation distance is computed from the Pearson correlation
  coefficient <code>r</code> as: <code>1-r</code>. Hence, possible values of
  this distance are between 0 and 2.
  
  @param v1 first feature vector
  @param v2 second feature vector
  @return the Pearson correlation distance for two given feature vectors v1
  and v2