Sheila Greibach coauthored the 1969 work on the abstract family of languages framework with Seymour Ginsburg and John Hopcroft. Their framework separates common language operations from the particular recognizers used to implement them.[ref]
Studies in Abstract Families of Languages by Seymour Ginsburg and others. Abstract families of languages, pp.1–32; collection byline
One important distinction is between nonerasing word homomorphisms and arbitrary word homomorphisms. Allowing an alphabet symbol to map to the empty word can discard arbitrarily much of a derivation's written evidence. A full abstract family of languages allows that operation, while the ordinary framework only requires the nonerasing version. The distinction prevents an apparently small change in a closure axiom from concealing a substantive change of computational behavior.
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Sheila Greibach is a prominent figure in the field of computer science, particularly known for her contributions to formal languages and automata theory. She has been recognized for her work in the area of grammars, specifically in relation to context-free grammars and their applications in computational linguistics and programming language design. Greibach is also known for the Greibach normal form, which is a specific way of representing context-free grammars.