code: public double variance(double m, Object pt, Object ptSqr) {
  		Object x = newCopy(ptSqr);
  //		// Var is E(X^2) - E(X)^2, but squaring pt1 introduces an extra factor which is the total mass, so:
  		subtractScaledSqr(x, 1 / m, pt);
  		return sum(x);
  	}
docs: ptSqr - aggregate point squared (prescaled by mass)