code: public void addCovariance( double[] difference , double responsibility ) {
  		int N = mean.numRows;
  		for (int i = 0; i < N; i++) {
  			for (int j = i; j < N; j++) {
  				covariance.data[i*N+j] += responsibility*difference[i]*difference[j];
  			}
  		}
  
  		for (int i = 0; i < N; i++) {
  			for (int j = 0; j < i; j++) {
  				covariance.data[i*N+j] = covariance.data[j*N+i];
  			}
  		}
  	}
docs: Helper function for computing Gaussian parameters.  Adds the difference between point and mean to covariance,
  adjusted by the weight.