docs: public void put(double[] val, double weight) {
      assert (val.length == elements.length);
      if(weight == 0) {
        return; // Skip zero weights.
      }
      final double nwsum = weight + wsum;
      for(int i = 0; i < elements.length; i++) {
        final double delta = val[i] - elements[i];
        final double rval = delta * weight / nwsum;
        elements[i] += rval;
      }
      wsum = nwsum;
    }