def box_at(row, col, width=3, height=3):
    """Return the box index of the field at (row, col)

    Args:
        row (int): The row of the field.
        col (int): The column of the field.
        width (int): The width of the sudoku.
        height (int): The height of the sudoku.

    Returns:
        int: The index of the box, in which the field (row, col) lies.
    """
    return col // width + row - (row % height)