= Seymour Ginsburg
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<Seymour Ginsburg> coauthored the 1969 studies of the <abstract family of languages> framework with <Sheila Greibach> and <John Hopcroft>. This approach compares <formal language>[formal-language classes] through closure properties instead of one particular grammar or machine presentation.<Studies in Abstract Families of Languages by Seymour Ginsburg and others>[Abstract families of languages, pp.1–32]{ref}
For example, closure under intersection with <regular language>[regular languages] permits restricting an existing language by a <finite-state automaton>[finite-state filter]. That property is weaker than closure under arbitrary intersections: two <context-free language>[context-free languages] can have a non-context-free intersection. Stating the allowed operation precisely keeps a shared abstract framework from incorrectly identifying different language classes.
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