def get_min_col_num(mention):
    """Return the lowest column 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.col_start
    else:
        return None