Do not use the qualified names like mathml:mn.
Instead use the prefixes mathml, local names like mn,
and URIs like http://www.w3.org/1998/Math/MathML
to create the elements.
Do not include xmlns attributes
like xmlns:mathml="http://www.w3.org/1998/Math/MathML".
XMLOutputter will
decide where to put the xmlns attributes
when the document is serialized.