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

		Returns
		-------
		pd.DataFrame, {'texts': <matching texts>, 'categories': <corresponding categories>}

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

		Returns
		-------
		pd.DataFrame, {'texts': <matching texts>, 'categories': <corresponding categories>}

		'''
		mask = self._document_index_mask(ngram)
		return pd.DataFrame({
			'text': self.get_texts()[mask],
			'category': [self._category_idx_store.getval(idx)
			             for idx in self._y[mask]]
		})