def colname(colx):
    """Utility function: ``7`` => ``'H'``, ``27`` => ``'AB'``"""
    alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    if colx <= 25:
        return alphabet[colx]
    else:
        xdiv26, xmod26 = divmod(colx, 26)
        return alphabet[xdiv26 - 1] + alphabet[xmod26]
