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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 clean_data(data):
 """ Shift to lower case, replace unknowns with UNK, and listify """

    new_data = []
    VALID = 'abcdefghijklmnopqrstuvwxyz123456789"\'?!.,:; '
    for sample in data:
        new_sample = []
        for char in sample[1].lower():  # Just grab the string, not the label
            if char in VALID:
                new_sample.append(char)
            else:
                new_sample.append('UNK')

        new_data.append(new_sample)
    return new_data