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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 append_id(expr, id_col='append_id'):
 """ Append an ID column to current column to form a new DataFrame. :param str id_col: name of appended ID field. :return: DataFrame with ID field :rtype: DataFrame """

    if hasattr(expr, '_xflow_append_id'):
        return expr._xflow_append_id(id_col)
    else:
        return _append_id(expr, id_col)