Practical Natural Language Processing: The REL System as Prototype

systems, t e x t s and grammars, b u t makes t h e concre te cases and examples hard t o f o l l o w . I n f$&t t h e r e a r e o t h e r u n i f o r m t reatments, mentioned below, which are undoubtedly b e t t e r f o r the p a r t i c u l a r cases i n ques t ion . The authors t r e a t neighborhood grammars i n these two chapters. A neighborhood of a ve r t ex i n a t r e e cons i s t s o f some o f t h e connect ing arcs and nearby nodes. For example, a neighborhood o f F i s (page 5.3): where t h e d i s t i n g u i s h e d node whose neighborhood i s i n ques t ion i s marked by Given a c o l l e c t i o n o f neighborhoods, a t r e e i s i n t he neighborhood language i f a1 1 t h e neighborhood c o n s t r a i n t s s p e c i f i e d i n fprmul a which c o n s t i t u t e the Mathematical Models of La qguage 8 3 grammar are sa t i s f i ed a1 1 nodes of the t r e e . The major virtue of thif approach i s i n e n a b l i i t h one t o specify other connections between the nodes o f t rees other than the usual descendant re la t ion . Thus i s a neighborhood specifying -via other information t o o complex t o describe here -t h & bo th copies of J dominate isomorphic subtrees. This enables one t o specify !'syntactically" t h a t every variable in a programming language m u s t be declared before i t i s used. However, there a re be t te r methods t o hand1 e these non-tree res t r ic t ions . For example, property grammars (Aho and Ullman, The Theory of Parsing, Transl a t ion , and Compiling: Vol. 2 , Compiling, Prentice-Hall,, Englewood Cl i f f s , N . J . , 1973), macro-grammars (M. 3 . Fi scher, Grammars with macro-1 i ke product ions . P h . D . Thesi s , Harvard Uni versi ty , 1968), and mathematical semantics ( R . D . Tennent, The Lnotat ional Semantics of Programmi ng Languages, Comm. ACM 19: 8 (Aug. 1976), 437-453. ) Chapter I I I , " c o n c r ~ t e s i gn sys terns" devel ops phrase s t ruc ture grammars 3nd nominal neighborhood grammars. The type 0 phrase s t ruc ture grammars produce "phrase s t ructures" , as generalizations o f t rees. iese phrase structures have appeared i n the Western 1 i tera ture i q W leas t the fbl lowing papers: J. Loeckx, The Parsing for General Phrase-Structure Grammars, Inform& Control 16:5 ( ~ u l 1970), 443-464, H. W . Buttelrnann, Oa the .Syntactic-Structures of Unres t r ic ted Grammars, I . Generative Grammars and Phrase Structure Grammars, Inform. & Control ZP: 1. (Sept 1975), 29-80, D. B . Benson, Syntax and Semantics: A categorical view, Inform. & Control 17 (1 970), 145-160, a1 1 three of which Mathema t i c a l Models of Language were o r i g i n a l l y w r i t t e n i n 1969-1970. This development was c l e a r l y r i p e a t t h a t t i m e i n Russia, Europe and t h e U. S . The nominal ---neicjhborhood -gralnmars a r e an extens ion o f ne i ghborhood grammars which a1 low f a i r l y complex s t r u c t u r e s . For example, t he f o l l ow ing i s taken froni page 90: I t appears t h a t the language n n n Iala2c-q In21 1 can be generated by nominal neighborhood grammars i n an essen t i a l l y con tex t f r e e manner. The norriinal neighborhood grammars a r e new t o me and appear t o o f f e r cons iderab le genera t ing power a t t hk usual expense o f a complex.