Deborah Dahl. <a href="http://www.w3.org/TR/1999/WD-voice-nlu-reqs-19991223"><cite>Natural Language Processing Requirements for Voice Markup Languages.</cite></a> 23 December 1999. W3C Working Draft. (Work in progress.) URL: <a href="http://www.w3.org/TR/1999/WD-voice-nlu-reqs-19991223">http://www.w3.org/TR/1999/WD-voice-nlu-reqs-19991223</a>