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    <abstract>Non-binary bottom-up constituency parsing commonly uses arity-specific reductions such as Reduce-X#k, which jointly specify the mother label and number of children. We show that arity need not be a primitive transition parameter. The proposed parser predicts the mother label separately and identifies the ordered child sequence from a delimiter-bounded stack configuration. In every well-formed reducible configuration, the active delimiter, label marker, and intervening completed items uniquely determine the reduction domain and hence its arity. This factorization removes label–arity-specific reduction actions while preserving direct construction of the original non-binary trees. Controlled experiments on PTB and CTB show that the delimiter-guided parser closely tracks an arity-specific baseline while using a substantially smaller action inventory. Its predicted trees retain the gold branching profile without systematic collapse at higher arities. The smaller inventory is obtained at the cost of longer derivations and boundary-sensitive errors involving delimiter insertion, preservation, and consumption.</abstract>
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%0 Conference Proceedings
%T Non-binary bottom-up constituency parsing without arity actions
%A Park, Jungyeul
%A Jo, Eunkyul Leah
%A Huang, Zihao
%Y Lee, Hung-Yi
%Y Dev, Sunipa
%Y Li, Jessy
%Y Elliott, Desmond
%S Findings of the Association for Computational Linguistics: EMNLP 2026
%D 2026
%8 October
%I Association for Computational Linguistics
%C Budapest, Hungary
%@ 979-8-89176-462-0
%F park-etal-2026-non
%X Non-binary bottom-up constituency parsing commonly uses arity-specific reductions such as Reduce-X#k, which jointly specify the mother label and number of children. We show that arity need not be a primitive transition parameter. The proposed parser predicts the mother label separately and identifies the ordered child sequence from a delimiter-bounded stack configuration. In every well-formed reducible configuration, the active delimiter, label marker, and intervening completed items uniquely determine the reduction domain and hence its arity. This factorization removes label–arity-specific reduction actions while preserving direct construction of the original non-binary trees. Controlled experiments on PTB and CTB show that the delimiter-guided parser closely tracks an arity-specific baseline while using a substantially smaller action inventory. Its predicted trees retain the gold branching profile without systematic collapse at higher arities. The smaller inventory is obtained at the cost of longer derivations and boundary-sensitive errors involving delimiter insertion, preservation, and consumption.
%U https://aclanthology.org/2026.findings-emnlp.611/
%P 12869-12879

Markdown (Informal)

[Non-binary bottom-up constituency parsing without arity actions](https://aclanthology.org/2026.findings-emnlp.611/) (Park et al., Findings 2026)

• Non-binary bottom-up constituency parsing without arity actions (Park et al., Findings 2026)

ACL

• Jungyeul Park, Eunkyul Leah Jo, and Zihao Huang. 2026. Non-binary bottom-up constituency parsing without arity actions. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 12869–12879, Budapest, Hungary. Association for Computational Linguistics.