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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 cross_join(df1, df2):
 """ Return a dataframe that is a cross between dataframes df1 and df2 ref: https://github.com/pydata/pandas/issues/5401 """

    if len(df1) == 0:
        return df2

    if len(df2) == 0:
        return df1

    # Add as lists so that the new index keeps the items in
    # the order that they are added together
    all_columns = pd.Index(list(df1.columns) + list(df2.columns))
    df1['key'] = 1
    df2['key'] = 1
    return pd.merge(df1, df2, on='key').loc[:, all_columns]