function _row(value, row) {
    // check dimensions
    if (value.size().length !== 2) {
      throw new Error('Only two dimensional matrix is supported');
    }

    validateIndex(row, value.size()[0]);
    var columnRange = range(0, value.size()[1]);
    var index = new MatrixIndex(row, columnRange);
    return value.subset(index);
  }
Retrieve a row of a matrix
@param {Matrix } value  A matrix
@param {number} row     The index of the row
@return {Matrix}        The retrieved row