Return information about documents in corpora:
        a list of tuples (doc_id, doc_title).

def catalog(self, categories=None):
        """
        Return information about documents in corpora:
        a list of tuples (doc_id, doc_title).
        """
        ids = self._filter_ids(None, categories)
        doc_meta = self._get_meta()
        return [(doc_id, doc_meta[doc_id].title) for doc_id in ids]