Task: Question Answering
Input (English): Context: In mathematics, computer science, and linguistics, a formal language is a set of strings of symbols together with a set of rules that are specific to it., Indexed languages are a class of formal languages discovered by Alfred Aho; they are described by indexed grammars and can be recognized by nested stack automata., Alfred Vaino Aho (born August 9, 1941) is a Canadian computer scientist best known for his work on programming languages, compilers, and related algorithms, and his textbooks on the art and science of computer programming., Indexed grammars are a generalization of context - free grammars in that nonterminals are equipped with lists of flags , or index symbols . The language produced by an indexed grammar is called an indexed language ., A context-free grammar (CFG) is a term used in formal language theory to describe a certain type of formal grammar. A context-free grammar is a set of production rules that describe all possible strings in a given formal language. Production rules are simple replacements. For example, the rule, Subject: indexed grammar, Relation: instance_of, Options: (A) august (B) class (C) computer (D) formal grammar (E) grammar (F) index (G) language (H) mathematics (I) programming (J) science
Output (English): formal grammar