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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 get_min_row_num(mention):
 """Return the lowest row number that a Mention occupies. :param mention: The Mention to evaluate. If a candidate is given, default to its first Mention. :rtype: integer or None """

    span = _to_span(mention)
    if span.sentence.is_tabular():
        return span.sentence.cell.row_start
    else:
        return None