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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _excel2num(x):
 """ Convert Excel column name like 'AB' to 0-based column index. Parameters x : str The Excel column name to convert to a 0-based column index. Returns ------- num : int The column index corresponding to the name. Raises ------ ValueError Part of the Excel column name was invalid. """

    index = 0

    for c in x.upper().strip():
        cp = ord(c)

        if cp < ord("A") or cp > ord("Z"):
            raise ValueError("Invalid column name: {x}".format(x=x))

        index = index * 26 + cp - ord("A") + 1

    return index - 1