Andrzej Ehrenfeucht Grzegorz Rozenberg Sven Skyum A Relationship between ET0L and EDT0L Languages. 325-330 1976 1 Theor. Comput. Sci. 4 db/journals/tcs/tcs1.html#EhrenfeuchtRS76 http://dx.doi.org/10.1016/0304-3975(76)90076-1