x in MarkupLanguage format.
http://forum.dlang.org/thread/awkynfizwqjnbilgddbh@forum.dlang.org#post-awkynfizwqjnbilgddbh:40forum.dlang.org https://developer.mozilla.org/en-US/docs/Web/MathML/Element/mn
https://developer.mozilla.org/en-US/docs/Web/MathML/Element/msup