def get_common_columns(df_dic):
    """
    :param: df_dic, a dictionary of data frames, as returned by 'get_all_dataframes'
    :return: a list of column names, representing the common subset of columns
            that appears in all data frames from 2013 on (2012 is excluded on purpose,
            since it has too few attributes)
    """
    years = list(df_dic.keys())
    # 2012 is years[0]
    y0 = years[1]
    years = years[2:]
    common = set(df_dic[y0].columns)
    for y in years:
        common = common.intersection(df_dic[y].columns)
    return sorted(common)