code: function reluB(product, left) {
    for (var i = 0; i < product.deltas.length; i++) {
      left.deltas[i] = left.weights[i] > 0 ? product.deltas[i] : 0;
    }
  }
docs: adds {from} deltas to {m} deltas when {m} weights are above other a threshold of 0
  @param {Matrix} product
  @param {Matrix} m