Eberhard Bergmann, R. Franck. Some concepts for languages more powerful than context-free. Computing, 10(4):289-304, 1972. [doi]
@article{BergmannF72,
title = {Some concepts for languages more powerful than context-free},
author = {Eberhard Bergmann and R. Franck},
year = {1972},
doi = {10.1007/BF02242242},
url = {http://dx.doi.org/10.1007/BF02242242},
researchr = {https://researchr.org/publication/BergmannF72},
cites = {0},
citedby = {0},
journal = {Computing},
volume = {10},
number = {4},
pages = {289-304},
}