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A context>free phrase structure general~or is in..~landard jb~'m if and only if alt of its rules are of the form: Z-, aY~, ... , Y,~ where Z and Yi are intermediate symbels and a is a l~erminM symbol, so that one input., symbol is processed at each step. Standard form is eonvenien(~ for computer manipulation of eontext-free languages. A proof is given that(More)
If a full AFLℒ is not closed under substitution, thenℒ ô ℒ, the result of substituting members ofℒ intoℒ, is not substitution closed and henceℒ generates an infinite hierarchy of full AFL's. Ifℒ 1 andℒ 2 are two incomparable full AFL's, then the least full AFL containingℒ 1 andℒ 2 is not substitution closed. In particular, the substitution closure of any(More)