docs: public void put(double x, double y, double w) {
      if(w == 0.) {
        return;
      }
      if(sumWe <= 0.) {
        sumX = x * w;
        sumY = y * w;
        sumWe = w;
        return;
      }
      // Delta to previous mean
      final double deltaX = x * sumWe - sumX;
      final double deltaY = y * sumWe - sumY;
      final double oldWe = sumWe;
      // Incremental update
      sumWe += w;
      final double f = w / (sumWe * oldWe);
      // Update
      sumXX += f * deltaX * deltaX;
      sumYY += f * deltaY * deltaY;
      // should equal weight * deltaY * neltaX!
      sumXY += f * deltaX * deltaY;
      // Update means
      sumX += x * w;
      sumY += y * w;
    }