AMR Similarity Metrics from Principles Juri Opitz author Letitia Parcalabescu author Anette Frank author 2020 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal opitz-etal-2020-amr 10.1162/tacl_a_00329 https://aclanthology.org/2020.tacl-1.34/ 2020 8 522 538