@inproceedings{EzqVilYliGomACL2025,
    title = "Hierarchical Bracketing Encodings for Dependency Parsing as Tagging",
    author = {Ezquerro, Ana  and
      Vilares, David  and
      Yli-Jyr{\"a}, Anssi  and
      G{\'o}mez-Rodr{\'i}guez, Carlos},
    editor = "Che, Wanxiang  and
      Nabende, Joyce  and
      Shutova, Ekaterina  and
      Pilehvar, Mohammad Taher",
    booktitle = "Proceedings of the 63rd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)",
    month = jul,
    year = "2025",
    address = "Vienna, Austria",
    publisher = "Association for Computational Linguistics",
    url = "https://aclanthology.org/2025.acl-long.903/",
    doi = "10.18653/v1/2025.acl-long.903",
    pages = "18436--18450",
    ISBN = "979-8-89176-251-0",
    abstract = "We present a family of encodings for sequence labeling dependency parsing, based on the concept of hierarchical bracketing. We show that the existing 4-bit projective encoding belongs to this family, but it is suboptimal in the number of labels used to encode a tree. We derive an optimal hierarchical bracketing, which minimizes the number of symbols used and encodes projective trees using only 12 distinct labels (vs. 16 for the 4-bit encoding). We also extend optimal hierarchical bracketing to support arbitrary non-projectivity in a more compact way than previous encodings. Our new encodings yield competitive accuracy on a diverse set of treebanks."
}