Masako Sato Yasuhito Mukouchi Dao Zheng Characteristic Sets for Unions of Regular Pattern Languages and Compactness. 220-233 http://link.springer.de/link/service/series/0558/bibs/1501/15010220.htm 1998 conf/alt/1998 ALT db/conf/alt/alt1998.html#SatoMZ98