Context-Limited Grammars

A phrase structure grammar is called context-limited if there exists a partial ordering on its alphabet such that any letter on the left of any production is less than some letter on the right of the same production. It is proved that context-limited grammars are equivalent to context-free grammars, the equivalence including ambiguity. The notion of ambiguity in phrase structure grammars is discussed, and a new formal model for ambiguity, based on directed plane graphs with labeled edges, is outlined and compared with other models.