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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 cummean(expr, sort=None, ascending=True, unique=False, preceding=None, following=None):
 """ Calculate cumulative mean of a sequence expression. :param expr: expression for calculation :param sort: name of the sort column :param ascending: whether to sort in ascending order :param unique: whether to eliminate duplicate entries :param preceding: the start point of a window :param following: the end point of a window :return: calculated column """

    data_type = _stats_type(expr)
    return _cumulative_op(expr, CumMean, sort=sort, ascending=ascending,
                          unique=unique, preceding=preceding,
                          following=following, data_type=data_type)