Returns
		-------
		PriorFactory

def drop_unused_terms(self):
		'''
		Returns
		-------
		PriorFactory
		'''
		self.term_doc_mat = self.term_doc_mat.remove_terms(
			set(self.term_doc_mat.get_terms()) - set(self.priors.index)
		)
		self._reindex_priors()
		return self