@article{AFM_1972_501-520_a2,
     author = {Timo Lepist\"o},
     title = {An extension for the concept of finite index of a context-free grammar},
     journal = {Annales Fennici Mathematici},
     publisher = {mathdoc},
     volume = {501-520},
     year = {1972},
     doi = {10.5186/aasfm.1972.518},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.5186/aasfm.1972.518/}
}