Do Long Van Nguyen Huong Lam Phan Trung Huy On Codes of Bi-Infinite Words. 97-110 1993 11 Acta Cybern. 1-2 http://www.inf.u-szeged.hu/kutatas/actacybernetica/vol11n12/dolong/dolong.xml db/journals/actaC/actaC11.html#VanLH93