Context-free Rewriting Research Articles

Data-oriented parsing with discontinuous constituents and function tags

Statistical parsers are e ective but are typically limited to producing projective dependencies or constituents. On the other hand, linguisti- cally rich parsers recognize non-local relations and analyze both form and function phenomena but rely on extensive manual grammar development. We combine advantages of the two by building a statistical parser that produces richer analyses. We investigate new techniques to implement treebank-based parsers that allow for discontinuous constituents. We present two systems. One system is based on a string-rewriting Linear Context-Free Rewriting System (LCFRS), while using a Probabilistic Discontinuous Tree Substitution Grammar (PDTSG) to improve disambiguation performance. Another system encodes the discontinuities in the labels of phrase structure trees, allowing for efficient context-free grammar parsing.The two systems demonstrate that tree fragments as used in tree-substitution grammar improve disambiguation performance while capturing non-local relations on an as-needed basis. Additionally, we present results of models that produce function tags, resulting in a more linguistically adequate model of the data. We report substantial accuracy improvements in discontinuous parsing for German, English, and Dutch, including results on spoken Dutch.

Simple restriction in context-free rewriting

Many rewriting systems with context-free productions and with controlled derivations have been studied. On one hand, these systems preserve the simplicity of applications of context-free productions and, on the other hand, they increase the generative power to cover more aspects of natural and programming languages. However, with λ-productions, many of these systems are computationally complete. It gives rise to a natural question of what are the simplest restrictions of the derivation process of context-free grammars to obtain the universal power. In this paper, we present such a simple restriction introducing so-called restricted context-free rewriting systems. These systems are context-free grammars with a function assigning a nonterminal coupled with + or − to each nonterminal. A production is applicable if it is applicable as a context-free production and if the symbol assigned to the left-hand side of the production is coupled with +, then this symbol has to appear in the sentential form, while if coupled with −, it must not appear in the sentential form. This restriction is simpler than most of the other restrictions, since the context conditions are assigned to nonterminals, not to productions, and their type is the simplest possible – a nonterminal.

Pattern control on derivations in context-free rewriting

Pattern control on derivations in context-free rewriting

On the Kleijn-Rozenberg k-adjacent languages

The k-adjacent derivations, which generate the k-adjacent languages, were introduced by Kleijn and Rozenberg as an intermediate rewriting process between context-free rewriting and EOL rewriting. In this paper, we study the generative power of the k-adjacent derivations, in continuation of the work of Gonczarowski and Shamir, and Dahlhaus and Gaifman. We show that these derivations generate languages which satisfy the following: 1. • for all k ⩾ 2, the family of the k-adjacent languages contains all EOL languages generated by expanding grammars (this is a generalization of the result of Dahlhaus and Gaifman where k = 2); 2. • for all k ⩾ 3, the family of the k-adjacent languages contains all ETOL languages generated by expanding grammars; 3. • for k > 2, ( k + l)-adjacency has the same generative power as k-adjacency if the productions right-hand sides are large enough; 4. • there are k-adjacent languages, k ⩾ 3, which are not ETOL.

An axiomatic definition of context-free rewriting and its application to NLC graph grammars

An abstract notion of context-free grammar is introduced. It deals with abstract objects that can be words, trees, graphs or other combinatorial objects. It is applied to NLC graph grammars introduced by Rozenberg and Janssens. The monadic second-order theory of a context-free NLC set of graphs is decidable.

On the complexity of regulated context-free rewriting

Some complexity measures which are well-known for context-free languages are generalized in order to classify matrix languages and programmed languages. It is shown that the complexity of some context-free languages decreases if they are generated by matrix grammars or programmed grammars. An arithmetic characterization is given for infinite languages generated by two matrices. The number of matrices (as a complexity measure) is shown to be independent from any other complexity measure regarded in this paper.