code: function add(product, left, right) {
    for (var i = 0; i < left.weights.length; i++) {
      product.weights[i] = left.weights[i] + right.weights[i];
      product.deltas[i] = 0;
    }
  }
docs: add {left} and {right} matrix weights into {into}
  @param {Matrix} product
  @param {Matrix} left
  @param {Matrix} right