@article{AhoIndexed,
 title = {Indexed Grammars—An Extension of Context-Free Grammars},
 author = {Aho, Alfred V.},
 year = {1968},
 doi = {10.1145/321479.321488},
 url = {https://doi.org/10.1145/321479.321488},
 journal = {J. ACM},
 volume = {15},
 number = {4},
 pages = {647–671},
 publisher = {Association for Computing Machinery},
 address = {New York, NY, USA},
 numpages = {25},
 month = {oct},
 abstract = {A new type of grammar for generating formal languages, called an indexed grammar, is presented. An indexed grammar is an extension of a context-free grammar, and the class of languages generated by indexed grammars has closure properties and decidability results similar to those for context-free languages. The class of languages generated by indexed grammars properly includes all context-free languages and is a proper subset of the class of context-sensitive languages. Several subclasses of indexed grammars generate interesting classes of languages.},
 issn = {0004-5411},
 issue_date = {Oct. 1968}
}