Problems in Current Text Simplification Research: New Data Can Help Wei Xu author Chris Callison-Burch author Courtney Napoles author 2015 text journal article Transactions of the Association for Computational Linguistics continuing MIT Press Cambridge, MA periodical academic journal xu-etal-2015-problems 10.1162/tacl_a_00139 https://aclanthology.org/Q15-1021/ 2015 3 283 297