def ensure_cell_exists(row_index, column_index = 0)
    validate_nonnegative(row_index)
    validate_nonnegative(column_index)

    sheet_data.rows[row_index] || add_row(row_index)
  end
def ensure_cell_exists(row_index, column_index = 0)
    validate_nonnegative(row_index)
    validate_nonnegative(column_index)

    sheet_data.rows[row_index] || add_row(row_index)
  end
Ensures that storage space for a cell with +row_index+ and +column_index+
 exists in +sheet_data+ arrays, growing them up if necessary.