# Context-free grammars: Covers, normal forms, and parsing by A. Nijholt

By A. Nijholt

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**Example text**

1. ]. 2. 2. Let fG' and h G be parse relations for grammars G' and G, respectively. If G'[f]h]G then G'[f/h]G. Proof. Let ~ be the cover homomorphism under which G'[f/h]G. Define ~R : AG ' AG such that, for any i e AG, , ~R(i) = ~[R if ~(i) = ~[. Then G'[f/h]G under cover homomorphism ~R. D Note, if the cover homomorphism ~ is fine then ~R and @ are identical on % , . Rather loosely formulated one can say that if a cover is supported t by rules of the form A ÷ a or A + e only, then we can treat left and right parses of the covering gra~mmr as being identical.

We show how the elimination of single productions can be done. We use auxiliary 42 sets P0' P! and P2" The set P0 is the set of all the single productions in P. Initially , P| = { A ÷ (i) ~ * ! i . A ÷ ~ is in P - P0 }, N' = N and P2 = ~" For any A £ N, if A ~ B ~ y is a derivation in G such that 6 # g and either IYI ~ 2 or y ~ S, then add [A6i] ÷ y <~> to P| and [A6i] to N'. To obtain a left cover, define ~ = 6i. To obtain a right cover, define ~ = i6 R. Notice that since G is cycle-free there are finitely many derivations to consider. *

We conclude this section with some notational remarks. In the following table some frequently used names and notations are displayed. We use £ to denote left parses (and left parse relations) and r to denote the right parses. In general, if f denotes a parse relation then f denotes the parse relation {(w,~) I (w, ~R) • f}" Apart from £,r,~ and r the abbreviations £p, standing for left part parses, and £c, standing for left corner parses, will sometimes be used. PARSE RELATION NOTATION NAME f f G'[f/h]G f-to-h cover left left G'[Z/Z]G left cover left i right i G'[£/~]G left-to-right cover right t left G'[~/£]G right-to-left cover G'[r/r]G right cover right right Table IV.