A GREIBACH NORMAL-FORM FOR CONTEXT-FREE GRAPH-GRAMMARS

Every context-free hypergraph grammar that generates a language of bounded degree can be transformed into an equivalent one that has the apex property, i.e., that cannot ''pass'' nodes from nonterminal to nonterminal. This generalizes Double Greibach Normal Form of context-free grammars. Moreover, it provides a natural grammatical characterization of the context-free hypergraph languages of bounded degree. For grammars with the apex property it is not possible to put a bound on the number of nonterminals in the right-hand sides of the productions.