Each regular code is included in a maximal regular code
RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 20 (1986) no. 1, pp. 89-96. 
@article{ITA_1986__20_1_89_0,
     author = {Ehrenfeucht, A. and Rozenberg, G.},
     title = {Each regular code is included in a maximal regular code},
     journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
     pages = {89--96},
     publisher = {AFCET - Gauthier-Villars},
     address = {Paris},
     volume = {20},
     number = {1},
     year = {1986},
     mrnumber = {849968},
     zbl = {0609.68053},
     language = {en},
     url = {http://www.numdam.org/item/ITA_1986__20_1_89_0/}
}
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Ehrenfeucht, A.; Rozenberg, G. Each regular code is included in a maximal regular code. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 20 (1986) no. 1, pp. 89-96. http://www.numdam.org/item/ITA_1986__20_1_89_0/

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