code: public double variance(double m1, Object pt1, Object ptSqr1, double m2, Object pt2, Object ptSqr2) {
  		// compute the total mass
  		double m0 = m1 + m2;
  		// compute the new sum
  		Object pt0 = newCopy(pt1);
  		add(pt0, pt2);
  		// compute the new sum of squares
  		Object ptSqr0 = newCopy(ptSqr1);
  		add(ptSqr0, ptSqr2);
  		// compute the variance
  		return variance(m0, pt0, ptSqr0);
  	}
docs: Note: (m1 + m2) never zero