Unsupervised Abstractive Opinion Summarization by Generating Sentences with Tree-Structured Topic Guidance Masaru Isonuma author Junichiro Mori author Danushka Bollegala author Ichiro Sakata author 2021 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal isonuma-etal-2021-unsupervised 10.1162/tacl_a_00406 https://aclanthology.org/2021.tacl-1.56/ 2021 9 945 961