LFG Generation from Acyclic F-Structures is NP-Hard Jürgen Wedekind author Ronald M Kaplan author 2021-12 text journal article Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal wedekind-kaplan-2021-lfg 10.1162/coli_a_00419 https://aclanthology.org/2021.cl-4.31/ 2021-12 47 4 939 946