[3] Pronunciation Lexicon Specification (PLS) Version 1.0 (2017-10-02 19:35:40 +09:00) <https://www.w3.org/TR/pronunciation-lexicon/>

[2] 音声認識文法の作成方法 — OpenHRI Manual (2015-09-26 15:10:32 +09:00) <http://openhri.readthedocs.io/en/latest/workingwithgrammar-ja.html>

[1] EPUB Content Documents 3.1 (2017-02-01 15:05:24 +09:00) <https://www.w3.org/Submission/2017/SUBM-epub-contentdocs-20170125/Overview.html#sec-pls-intro>