def _prepend_row_index(rows, index):
    """Add a left-most index column."""
    if index is None or index is False:
        return rows
    if len(index) != len(rows):
        print('index=', index)
        print('rows=', rows)
        raise ValueError('index must be as long as the number of data rows')
    rows = [[v] + list(row) for v, row in zip(index, rows)]
    return rows
Add a left-most index column.