Bracketing Encodings for 2-Planar Dependency Parsing

Michalina Strzyz, David Vilares, Carlos Gómez-Rodríguez


Abstract
We present a bracketing-based encoding that can be used to represent any 2-planar dependency tree over a sentence of length n as a sequence of n labels, hence providing almost total coverage of crossing arcs in sequence labeling parsing. First, we show that existing bracketing encodings for parsing as labeling can only handle a very mild extension of projective trees. Second, we overcome this limitation by taking into account the well-known property of 2-planarity, which is present in the vast majority of dependency syntactic structures in treebanks, i.e., the arcs of a dependency tree can be split into two planes such that arcs in a given plane do not cross. We take advantage of this property to design a method that balances the brackets and that encodes the arcs belonging to each of those planes, allowing for almost unrestricted non-projectivity (∼99.9% coverage) in sequence labeling parsing. The experiments show that our linearizations improve over the accuracy of the original bracketing encoding in highly non-projective treebanks (on average by 0.4 LAS), while achieving a similar speed. Also, they are especially suitable when PoS tags are not used as input parameters to the models.
Anthology ID:
2020.coling-main.223
Volume:
Proceedings of the 28th International Conference on Computational Linguistics
Month:
December
Year:
2020
Address:
Barcelona, Spain (Online)
Editors:
Donia Scott, Nuria Bel, Chengqing Zong
Venue:
COLING
SIG:
Publisher:
International Committee on Computational Linguistics
Note:
Pages:
2472–2484
Language:
URL:
https://aclanthology.org/2020.coling-main.223
DOI:
10.18653/v1/2020.coling-main.223
Bibkey:
Cite (ACL):
Michalina Strzyz, David Vilares, and Carlos Gómez-Rodríguez. 2020. Bracketing Encodings for 2-Planar Dependency Parsing. In Proceedings of the 28th International Conference on Computational Linguistics, pages 2472–2484, Barcelona, Spain (Online). International Committee on Computational Linguistics.
Cite (Informal):
Bracketing Encodings for 2-Planar Dependency Parsing (Strzyz et al., COLING 2020)
Copy Citation:
PDF:
https://aclanthology.org/2020.coling-main.223.pdf
Code
 mstrise/dep2label