Parameters
        ----------
        ngram, str or unicode, string to search for

        Returns
        -------
        pd.DataFrame, {self._parsed_col: <matching texts>, self._category_col: <corresponding categories>, ...}

def search(self, ngram):
        '''
        Parameters
        ----------
        ngram, str or unicode, string to search for

        Returns
        -------
        pd.DataFrame, {self._parsed_col: <matching texts>, self._category_col: <corresponding categories>, ...}

        '''
        mask = self._document_index_mask(ngram)
        return self._df[mask]