Abstract

Define a cylinder to be a family of languages which is closed under inverse homomorphisms and intersection with regular sets. A number of well-known families of languages are cylinders: —CFL, the family of context-free languages, is a principal cylinder, i.e. the smallest cylinder containing a languageLO described in [6]. —the family of deterministic context-free languages is proved to be a nonprincipal cylinder in [7]. —the family of unambiguous context-free languages is a cylinder: to prove that it is not principal seems to be a very hard problem.

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