Parameters
        ----------
        background

def set_background_corpus(self, background):
        '''
        Parameters
        ----------
        background

        '''
        if issubclass(type(background), TermDocMatrixWithoutCategories):
            self._background_corpus = pd.DataFrame(background
                                                   .get_term_freq_df()
                                                   .sum(axis=1),
                                                   columns=['background']).reset_index()
            self._background_corpus.columns = ['word', 'background']
        elif (type(background) == pd.DataFrame
              and set(background.columns) == set(['word', 'background'])):
            self._background_corpus = background
        else:
            raise Exception('The argument named background must be a subclass of TermDocMatrix or a ' \
                            + 'DataFrame with columns "word" and "background", where "word" ' \
                            + 'is the term text, and "background" is its frequency.')