def column_letter_to_index(column: str) -> int:
    """
    used exclusively in conversions
    This function converts a letter column to its respective 0-indexed column index
    viz. 'A' to 0
    'AZ' to 51

    Returns:
        int: column index
    """
    index = 0
    column = column.upper()
    column = column[::-1]
    for i in range(len(column)):
        index += ((ord(column[i]) % 65 + 1) * (26 ** i))
    return index - 1