Grammar Schemata

A solution is presented for the following problem: Determine a procedure that produces, for each full trio L of context-free languages (more generally, each trio of r.e. languages), a family of context-free (phrase structure) grammars which (a) defines L, (b) is simple enough for practical and theoretical purposes, and (c) in most cases is a subfamily of a well-known family of context-free (phrase structure) grammars for L if such a well-known family exists. (A full trio (trio) is defined to be a family of languages closed under homomorphism (ε-free homomorphism), inverse homomorphism, and intersection with regular sets.) The key notion in the paper is that of a grammar schema. With each grammar schema there is associated a family of interpretations. In turn, each interpretation of a grammar schema gives rise to a phrase structure grammar. Given a full trio (trio) L of context-free (r.e.) languages, one constructs a grammar schema whose interpretations (ε-limited interpretations) then give rise to the desired family of grammars for L.