= Sheila Greibach
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= Sheila A. Greibach
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<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.<Studies in Abstract Families of Languages by Seymour Ginsburg and others>[Abstract families of languages, pp.1–32; collection byline]{ref}
One important distinction is between nonerasing word <monoid homomorphism>[homomorphisms] and arbitrary word <monoid homomorphism>[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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